Chagos Islands Deal Review: What International Law Says
Introduction
The Chagos Islands deal remains signed but has not entered into force. The United Kingdom and Mauritius concluded the agreement on 22 May 2025, but as of 23 September 2026 the UK has not ratified it, and the legislation intended to implement the settlement did not survive the end of the 2024–26 parliamentary session (Foreign, Commonwealth & Development Office, 2025; UK Parliament, 2026a; UK Parliament, 2026b). The present review is the latest stage of an impasse that had already stalled progress by April 2026. President Donald Trump’s renewed criticism of the agreement during his September meeting with Prime Minister Andy Burnham intensified an existing disagreement over the future of Diego Garcia rather than creating it (UK Parliament, 2026c; Reuters, 2026a; Reuters, 2026b).
If brought into force, the treaty would recognize Mauritius as sovereign over the entire Chagos Archipelago, including Diego Garcia. At the same time, it would authorize the United Kingdom to exercise extensive rights and authorities necessary for the operation and security of the military base, including continued cooperation with the United States. Those arrangements would initially last for 99 years from the treaty’s entry into force (United Kingdom and Mauritius, 2025). The legal basis of British authority on Diego Garcia would consequently change: the United Kingdom would cease to rely on its own claim to territorial sovereignty and would instead exercise defined powers under an agreement with Mauritius. That treaty-based arrangement is not yet operative.
The distinction between signature and entry into force is central to the current dispute. Under Article 18 of the Vienna Convention on the Law of Treaties, a state that has signed a treaty subject to ratification must refrain from acts that would defeat its object and purpose until it makes clear that it does not intend to become a party (Vienna Convention on the Law of Treaties, 1969). This interim obligation does not make the entire treaty binding before entry into force, nor does signature itself impose a general duty to ratify. Provisional application is a separate legal mechanism governed by Article 25 of the Convention (Aust, 2005; Klabbers, 2024).
The treaty’s uncertain future does not erase the international-law dispute that preceded it. In its 2019 advisory opinion, the International Court of Justice concluded that the decolonization of Mauritius had not been lawfully completed following the separation of the Chagos Archipelago and that the United Kingdom was obliged to end its administration of the territory as rapidly as possible (ICJ, 2019). The UN General Assembly subsequently called for completion of the decolonization of Mauritius in Resolution 73/295 (UN General Assembly, 2019). In 2021, the Special Chamber of the International Tribunal for the Law of the Sea addressed the consequences of the Chagos dispute when rejecting preliminary objections in the Mauritius–Maldives maritime delimitation proceedings (ITLOS, 2021).
A British decision to revise, replace, or ultimately abandon the 2025 agreement would decide the fate of one negotiated settlement, not the entire legal status of Chagos. The ICJ did not prescribe the terms of the 2025 treaty as the exclusive method of completing decolonization, but neither would the failure of that treaty remove the legal conclusions reached in 2019. The immediate question is thus narrower and more difficult: how far the United Kingdom may reconsider a signed but non-operative treaty while complying with the law of treaties, and how any resulting settlement would interact with the continuing questions of sovereignty, Diego Garcia, and the rights and interests of the Chagossian community.
1. Is the Chagos Islands Deal Dead?
The Chagos Islands deal is not legally “dead” as of 23 September 2026. The agreement remains signed but has not entered into force, and the current parliamentary record shows that scrutiny has been completed without recording ratification as having occurred. The political position has deteriorated sharply, however. The legislation intended to implement the settlement in UK law did not complete its passage, President Donald Trump reversed his earlier support for the arrangement, and the Burnham government is now reviewing the agreement in circumstances where US political backing is no longer available (UK Parliament, 2026a; Reuters, 2026a; Sky News, 2026).
These developments must be distinguished from the legal acts that could end the present treaty process. Review is not ratification, but neither is it formal abandonment. A decision not to ratify differs from termination of a treaty already in force, while a clear declaration that the United Kingdom does not intend to become a party would have consequences under Article 18 of the Vienna Convention on the Law of Treaties. The current public record does not establish that such a declaration has been made (Vienna Convention on the Law of Treaties, 1969, art. 18).
The more accurate description is that the agreement remains a signed but non-operative treaty whose future is under reconsideration. Shortly before the latest review, the UK Government was still referring to continuing engagement with Mauritius on the treaty, while the September discussions with Washington concerned whether a politically workable arrangement could still be found for Diego Garcia (UK Parliament, 2026b; Reuters, 2026b). The legal position is uncertain, but uncertainty is not the same as extinction.
1.1 What Happened to the Chagos Islands Bill?
The Diego Garcia Military Base and British Indian Ocean Territory Bill was intended to implement in UK law the domestic changes required by the treaty. Among other things, it would have ended the existing constitutional status of the British Indian Ocean Territory, provided for the continuation of British administration on Diego Garcia under the new treaty arrangements, and amended relevant nationality provisions (UK Parliament, 2025a).
The Bill passed through substantial stages in Parliament but did not receive Royal Assent. The 2024–26 parliamentary session was prorogued on 29 April 2026, after which Parliament recorded that the Bill would make no further progress (UK Parliament, 2026c). That outcome did not amount to a parliamentary rejection of the treaty itself. The separate scrutiny period under the Constitutional Reform and Governance Act 2010 had already expired in July 2025 without either House adopting a resolution preventing ratification (House of Commons Library, 2025).
The distinction reflects a basic feature of the UK constitutional system. Treaty-making is conducted internationally by the executive, but treaties do not ordinarily change domestic law merely through signature or ratification. Where implementation requires changes to statutory rights, territorial arrangements, or other matters governed by domestic law, Parliament must legislate. The Supreme Court reaffirmed that constitutional separation in R (Miller) v Secretary of State for Exiting the European Union [2017] UKSC 5, emphasizing that ministers cannot use the treaty prerogative itself to alter domestic law.
The failure of the Bill consequently interrupted the domestic route toward implementation without cancelling the signed agreement. Article 18 of the Chagos treaty makes entry into force dependent on the parties notifying one another that their respective internal requirements and procedures have been completed (United Kingdom and Mauritius, 2025, art. 18). If the United Kingdom were later to make clear that it no longer intended to become a party, that would affect its interim obligations under international treaty law, but it would not erase the fact that the 2025 instrument was negotiated, authenticated, and signed.
2. What the 2025 Chagos Treaty Would Change
The treaty’s central provision is Article 1, which states that “Mauritius is sovereign over the Chagos Archipelago in its entirety, including Diego Garcia” (United Kingdom and Mauritius, 2025, art. 1). If the agreement enters into force, the United Kingdom would cease to maintain its competing claim of sovereignty over the archipelago. British authority on Diego Garcia would instead rest on extensive rights and powers granted under the treaty by Mauritius.
That distinction separates sovereignty from the exercise of governmental and military functions. Article 2 authorizes the United Kingdom to exercise the rights and authorities required for the long-term operation of the base. Article 3 assigns the United Kingdom full responsibility for the defense and security of Diego Garcia, while Article 9 regulates jurisdiction and control. Mauritius would retain sovereign title while the United Kingdom exercised defined powers for the specific purposes created by the agreement (United Kingdom and Mauritius, 2025, arts. 2, 3 and 9).
The arrangement also preserves the American military presence. The treaty permits the United Kingdom to authorize continued joint operation of the base with the United States, and Annex 1 provides extensive rights concerning access, basing, overflight, military personnel, weapons, communications, security, administration, and policing. The legal structure is consequently more extensive than a conventional commercial lease and is designed to preserve a high degree of British operational authority on Diego Garcia despite Mauritian sovereignty (United Kingdom and Mauritius, 2025, Annex 1).
Article 13 provides for an initial term of 99 years beginning when the treaty enters into force. A further 40-year period may be agreed between the parties, and additional extensions are possible by agreement. The additional 40 years are not automatic. If no extension is agreed after the initial term, the treaty provides for a right of first refusal for the United Kingdom in relation to future use of Diego Garcia on equivalent terms offered to another state or third party (United Kingdom and Mauritius, 2025, art. 13).
The agreement is thus better understood as a negotiated reallocation of legal authority than as a simple transfer of a military base. It would establish bilateral recognition of Mauritian sovereignty while preserving substantial British and American operational rights. The change concerns the legal foundation of British authority: instead of exercising powers on the basis of a British sovereignty claim, the United Kingdom would exercise them under a treaty with Mauritius.
3. What Does Signing the Treaty Legally Mean?
Signature, ratification, domestic approval, and entry into force are legally distinct stages. The United Kingdom and Mauritius signed the Chagos agreement on 22 May 2025, but signature did not make its substantive provisions immediately operative. The published text identifies the agreement as not yet in force, and Article 18 requires a later exchange of notifications confirming completion of the parties’ internal procedures before the treaty becomes effective (United Kingdom and Mauritius, 2025, art. 18).
In the United Kingdom, the agreement was laid before Parliament under the Constitutional Reform and Governance Act 2010. The statutory scrutiny period expired without Parliament resolving against ratification, leaving the Government legally able to proceed under the domestic treaty-scrutiny framework. That did not itself constitute ratification, nor did it bring the treaty into force (House of Commons Library, 2025).
The distinction is also important at the international level. Ratification is an act by which a state expresses consent to be bound, while entry into force depends on the conditions set by the treaty itself. A state may complete its domestic approval process without the treaty yet becoming operative internationally. In the Chagos agreement, entry into force is tied to reciprocal notification of completed internal requirements and occurs on the date specified by Article 18.
Article 26 of the Vienna Convention provides that every treaty in force is binding upon the parties and must be performed in good faith. The Chagos agreement has not yet reached that stage. Its provisions on sovereignty, jurisdiction, security arrangements, payments, and the 99-year term cannot be treated as fully operative treaty obligations merely because the agreement has been signed (Vienna Convention on the Law of Treaties, 1969, art. 26).
3.1 Article 18 of the Vienna Convention
The absence of entry into force does not mean that signature is without legal effect. Both the United Kingdom and Mauritius are parties to the Vienna Convention on the Law of Treaties: the United Kingdom ratified it in 1971 and Mauritius acceded in 1973 (United Nations Treaty Collection, n.d.). Article 18 consequently governs their conduct during the period before the Chagos agreement enters into force or one party makes clear that it does not intend to become bound.
Article 18 requires a state that has signed a treaty subject to ratification, acceptance, or approval to refrain from acts that would defeat the treaty’s object and purpose until it has made clear its intention not to become a party. The rule is narrower than a duty to perform the treaty itself. It does not convert every substantive provision into an immediately binding obligation, nor does it create a general duty to complete ratification (Vienna Convention on the Law of Treaties, 1969, art. 18; Aust, 2005; Klabbers, 2024).
Provisional application is a separate mechanism. Article 25 of the Vienna Convention allows a treaty, or part of it, to be applied provisionally where the treaty so provides or the negotiating states otherwise agree. The Chagos agreement contains no provision establishing provisional application. Its territorial and military regime is consequently not operating simply because the instrument has been signed (Vienna Convention on the Law of Treaties, 1969, art. 25).
Applied to the current review, Article 18 does not prevent the United Kingdom from reconsidering whether it wishes to become bound. An internal review, diplomatic consultations, or a proposal to amend the agreement is not in itself equivalent to defeating the treaty’s object and purpose. Any allegation of breach would depend on the specific conduct at issue and whether, assessed in context, that conduct frustrated the agreement’s object and purpose. The scope of that interim obligation remains narrower, and less precisely defined, than full treaty performance (Aust, 2005).
3.2 Can Britain Review or Renegotiate the Agreement?
The United Kingdom may review the agreement and may seek renegotiation with Mauritius. Neither the treaty nor the Vienna Convention requires a government to preserve unchanged every political position taken at signature. Any amendment to the bilateral agreement, however, requires Mauritian consent. Article 16 provides that amendments must be agreed between the parties and made in writing (United Kingdom and Mauritius, 2025, art. 16).
Several legally distinct outcomes remain possible. The parties could proceed with the existing agreement, negotiate amendments, replace it with a new settlement, or decide not to continue with the treaty process. Britain may propose different terms, but it cannot unilaterally alter the rights and obligations contained in the signed text.
If the United Kingdom ultimately makes clear that it does not intend to become a party, Article 18(a) VCLT provides that the interim obligation not to defeat the treaty’s object and purpose ends at that point. Such a declaration would not retrospectively eliminate any legal consequences arising from earlier conduct, nor would it erase the signed text as a historical legal instrument. It would instead mark the end of the United Kingdom’s existing path toward becoming bound by that agreement.
As of 23 September 2026, the public record does not establish that this stage has been reached. The Government has described the agreement as under review and has continued to refer to engagement with Mauritius. Renegotiation, replacement, and non-ratification remain possible outcomes rather than settled conclusions (UK Parliament, 2026b; Sky News, 2026).
4. Who Owns the Chagos Islands Now?
The question “Who owns the Chagos Islands?” is better expressed in international law as a question of sovereignty and administration. The United Kingdom continues to administer the archipelago through the British Indian Ocean Territory, which remained listed by the Foreign, Commonwealth & Development Office as a British Overseas Territory in 2026 (Foreign, Commonwealth & Development Office, 2026). That administrative fact does not resolve the international-law status of the territory.
The historical sequence is central. Chagos formed part of the non-self-governing territory of Mauritius under British colonial administration. On 8 November 1965, the United Kingdom detached the archipelago and created the British Indian Ocean Territory. Mauritius became independent on 12 March 1968 (ICJ, 2019).
In 2019, the International Court of Justice concluded that the separation had not been based on the free and genuine expression of the will of the people concerned. The Court held that the United Kingdom, as administering power, was required to respect the territorial integrity of Mauritius during the decolonization process and that detaching Chagos prevented that process from being lawfully completed. It characterized continued British administration as a wrongful act of a continuing character (ICJ, 2019, paras. 173–178).
The present position consequently contains two distinct elements. The United Kingdom administers the islands in fact and under its domestic legal arrangements, but the ICJ concluded that continuation of that administration is internationally wrongful in the context of Mauritian decolonization. The 2025 treaty would resolve the bilateral sovereignty dispute by recognizing Mauritian sovereignty and establishing a new legal basis for continued British military authority on Diego Garcia. Its failure to enter into force has not displaced the international-law conclusions reached before the treaty was signed.
5. What the ICJ Decided on Chagos in 2019
The International Court of Justice delivered its advisory opinion on the Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965 on 25 February 2019. The proceedings followed UN General Assembly Resolution 71/292, which asked whether the decolonization of Mauritius had been lawfully completed when independence was granted in 1968 after the separation of Chagos and what legal consequences arose from the United Kingdom’s continued administration of the archipelago (ICJ, 2019).
The Court placed self-determination at the center of its reasoning. It concluded that the right to self-determination had crystallized as a rule of customary international law applicable to the decolonization of Mauritius. For non-self-governing territories, the Court treated territorial integrity as a corollary of that right and held that an administering power could not lawfully detach part of such a territory unless the separation reflected the freely expressed and genuine will of the people concerned (ICJ, 2019).
That requirement was not satisfied in 1965. Mauritius remained under British colonial rule when the Lancaster House arrangements were accepted, and the Court did not regard those arrangements as an international agreement between two independent states. It concluded that the detachment had not resulted from a free and genuine expression of the relevant will and held, by thirteen votes to one, that Mauritian decolonization had not been lawfully completed when independence was granted in 1968 (ICJ, 2019, paras. 170–174, 183).
The Court also identified continuing legal consequences. It held that the United Kingdom’s continued administration of the Chagos Archipelago constituted a wrongful act of a continuing character and that the United Kingdom was under an obligation to bring that administration to an end “as rapidly as possible.” By the same thirteen-to-one majority, it concluded that all UN member states were obliged to cooperate with the United Nations in completing the decolonization of Mauritius. The Court left the modalities for completing that process to the General Assembly rather than prescribing a specific territorial settlement (ICJ, 2019, paras. 177–183).
5.1 Did the ICJ Award Chagos to Mauritius?
The ICJ did not “award” Chagos to Mauritius in the form of a conventional territorial-title judgment. The 2019 proceeding was advisory, initiated by the UN General Assembly, rather than contentious litigation brought by Mauritius against the United Kingdom on the basis of consent to the Court’s jurisdiction. The questions submitted to the Court concerned decolonization and the legal consequences of continued British administration.
That procedural distinction does not diminish the substance of what the Court decided. It found that Chagos had formed part of the non-self-governing territory of Mauritius, that its detachment was incompatible with the applicable law of self-determination, and that continued British administration was internationally wrongful. It also held that the administering power was required to respect the territorial integrity of Mauritius, including the Chagos Archipelago (ICJ, 2019, paras. 173–178).
The precise formulation is that the Court did not transfer sovereignty by judicial decree. It determined that the colonial separation was unlawful within the law of decolonization and that the resulting British administration could not lawfully continue. The 2025 treaty is a later negotiated settlement through which the United Kingdom and Mauritius sought to resolve the bilateral consequences of that position while preserving the military base on Diego Garcia.
5.2 Is the Advisory Opinion Legally Binding?
An ICJ advisory opinion is not formally binding in the same manner as a judgment delivered in contentious proceedings between states. Advisory jurisdiction allows the Court to answer legal questions submitted by authorized UN organs and agencies, and the resulting opinion does not create res judicata between states in the way a contentious judgment does (ICJ, n.d.).
That formal distinction does not mean that the legal rules identified by the Court are optional. The Court did not create the right of self-determination in 2019; it determined that self-determination was part of the applicable customary international law. Its findings concerning the unlawfulness of the 1965 detachment and the continuing character of British administration were applications of those legal rules to the Chagos situation (ICJ, 2019).
The Court also expressed concrete legal consequences. It stated that the United Kingdom was under an obligation to end its administration as rapidly as possible and that all UN member states had an obligation to cooperate with the United Nations in completing Mauritian decolonization. Those operative conclusions were adopted by thirteen votes to one, with Judge Donoghue dissenting from the central findings (ICJ, 2019).
Describing the opinion simply as “non-binding” would thus be incomplete. The advisory opinion itself does not have the same formal binding effect as a contentious judgment, but it remains an authoritative judicial statement by the principal judicial organ of the United Nations on the customary international law governing the separation and continued administration of Chagos.
6. How the UN and ITLOS Treated the Chagos Dispute
The 2019 advisory opinion did not end the Chagos dispute institutionally. Its legal conclusions were subsequently taken up by the UN General Assembly and, two years later, by the Special Chamber of the International Tribunal for the Law of the Sea. Those bodies exercised different functions and produced acts with different legal effects. The General Assembly responded within the UN decolonization framework, while the Special Chamber confronted the consequences of the Chagos dispute in contentious proceedings under the law of the sea.
The distinction is important. The General Assembly could determine how the United Nations should respond to the incomplete decolonization identified by the ICJ, but its resolution was not itself a judicial judgment settling a bilateral territorial dispute. ITLOS, by contrast, had to decide whether the legal status of Chagos prevented it from exercising jurisdiction in a maritime-boundary case between Mauritius and Maldives.
The sequence shows how the international position developed after 2019. The ICJ identified the applicable legal rules and consequences; the General Assembly addressed completion of the decolonization process; and the Special Chamber later assessed the implications of those developments for a dispute under UNCLOS. Their significance is cumulative, but their legal authority is not interchangeable.
6.1 UN General Assembly Resolution 73/295
On 22 May 2019, the General Assembly adopted Resolution 73/295 by 116 votes to six, with 56 abstentions. The resolution welcomed the ICJ advisory opinion and affirmed, consistently with it, that the decolonization of Mauritius had not been lawfully completed and that the Chagos Archipelago formed an integral part of the territory of Mauritius (UN General Assembly, 2019).
The Assembly demanded that the United Kingdom withdraw its colonial administration from the archipelago unconditionally within six months. It also called on member states and international organizations to cooperate with the United Nations in completing Mauritian decolonization and urged the United Kingdom to cooperate with Mauritius in facilitating resettlement, including for those of Chagossian origin (UN General Assembly, 2019, paras. 3–6).
Resolution 73/295 did not have the same legal character as the ICJ opinion on which it relied. General Assembly resolutions are generally recommendatory rather than binding in themselves, subject to specific powers conferred by the UN Charter. In the Chagos proceedings, however, the ICJ had expressly stated that the General Assembly should determine the modalities necessary to complete the decolonization of Mauritius. The resolution consequently carried particular institutional significance even though it was not itself a binding judicial decision.
6.2 The Mauritius–Maldives Judgment at ITLOS
The next major judicial development arose in the maritime-boundary dispute between Mauritius and Maldives. Mauritius sought delimitation of their exclusive economic zones and continental shelves, while Maldives argued, among other preliminary objections, that the unresolved Chagos sovereignty issue prevented the Special Chamber from proceeding and that the United Kingdom was an indispensable third party (ITLOS, 2021).
The Special Chamber rejected that position. It acknowledged that ICJ advisory opinions are not binding as such but distinguished formal binding force from the legal significance of judicial determinations. The Chamber considered that the findings made by the ICJ had legal effect and clear implications for the status of the Chagos Archipelago (ITLOS, 2021, paras. 202–206, 246).
In applying those findings, the Chamber concluded that the United Kingdom’s continued sovereignty claim was contrary to the determinations made by the ICJ and stated that Mauritian sovereignty over the Chagos Archipelago could be inferred from them. It also regarded the expiry of the six-month period specified in Resolution 73/295 as relevant when assessing the continuing British position (ITLOS, 2021, paras. 227–229, 246).
The relevant preliminary objection was rejected by eight votes to one. Judge ad hoc Oxman dissented, questioning the Chamber’s treatment of the territorial issue and the implications of proceeding without the United Kingdom. The majority nonetheless concluded that the Chagos question did not prevent it from exercising jurisdiction and treated Mauritius as the relevant coastal state for purposes of the maritime delimitation dispute (ITLOS, 2021, paras. 247–251).
The judgment did not itself transfer sovereignty over Chagos to Mauritius. Its significance lies elsewhere: a judicial body deciding a contentious UNCLOS dispute treated the ICJ findings and subsequent UN response as sufficient to overcome the argument that the continuing British claim made maritime delimitation legally impossible.
7. Diego Garcia, Trump, and the US Role
The United States is not a party to the 2025 UK–Mauritius agreement. Nothing in that treaty gives Washington a formal legal power to approve, reject, or amend its provisions. American influence instead derives from the separate and longstanding UK-US defense relationship centered on Diego Garcia.
That relationship originated in the 1966 Exchange of Notes, under which the United Kingdom made the British Indian Ocean Territory available for the defense purposes of both governments. The arrangement was later supplemented and was extended in 2016 for another 20 years, to December 2036. The proposed settlement with Mauritius was designed to coexist with this separate bilateral defense framework.
The interaction between the two regimes became critical during the ratification process. In February 2026, the UK Government stated that the UK-US agreement was being updated and that the revised arrangement would be finalized before the UK–Mauritius treaty was ratified (UK Parliament, 2026a). American agreement was thus not a legal condition written into the Mauritius treaty, but the British Government treated it as a practical prerequisite for proceeding with the new regime.
That difficulty intensified after President Donald Trump reversed his earlier support for the settlement. During his meeting with Prime Minister Andy Burnham on 22 September 2026, Trump again criticized the existing agreement. On 23 September, Defence Secretary Wes Streeting confirmed that the Government was reviewing the deal and stated that it could not proceed in its existing form without US support (Reuters, 2026; Sky News, 2026).
The treaty itself explains the operational importance of Washington’s position. Diego Garcia is subject to extensive arrangements concerning military access, basing, overflight, weapons, personnel, communications, jurisdiction, security, and operational control. A UK–Mauritius agreement can establish the territorial and sovereignty framework, but the intended military regime depends in practice on continued UK-US cooperation. US opposition is consequently a major operational and political obstacle, not a formal veto over a bilateral treaty between the United Kingdom and Mauritius.
8. Do Chagossians Have a Right of Return?
The 2025 treaty does not create an unconditional individual right for Chagossians to return throughout the archipelago. Article 6 provides that Mauritius is free, in the exercise of its sovereignty, to implement a program of resettlement on the Chagos Islands other than Diego Garcia, in accordance with Mauritian law and the agreement (United Kingdom and Mauritius, 2025, art. 6).
The provision creates a legal route through which Mauritius could establish resettlement on the outer islands, but its limits are clear. Diego Garcia is expressly excluded. Nor does Article 6 give every Chagossian an immediately enforceable treaty entitlement to live on any island of choice. Article 11 separately provides for a trust fund for the benefit of Chagossians, but that financial commitment is distinct from any resettlement entitlement (United Kingdom and Mauritius, 2025, arts. 6 and 11).
The ICJ did not hold in 2019 that Chagossians possess an individually enforceable right to return to Diego Garcia. The Court instead characterized resettlement as an issue concerning the protection of the human rights of those affected and stated that it should be addressed by the General Assembly during completion of the decolonization process (ICJ, 2019, para. 181). Resolution 73/295 subsequently called on the United Kingdom to cooperate with Mauritius in facilitating resettlement and to place no obstacle in the way of that process (UN General Assembly, 2019).
That conclusion does not exhaust every possible legal claim concerning return. The 2025 treaty, the ICJ advisory opinion, and Resolution 73/295 answer different legal questions. None of them, by itself, determines the full scope of any rights that might arise independently under international human rights law or domestic law. What can be said with confidence is narrower: the 2025 agreement does not itself establish a general individual right of return, and the ICJ did not recognize an enforceable right to return specifically to Diego Garcia.
Chagossian opinion on the settlement is also not uniform. The House of Lords International Relations and Defence Committee recorded divergent views concerning sovereignty, resettlement, consultation, and the treaty itself. Its survey received more than 3,000 responses but expressly cautioned that the respondents were self-selecting and that the results could not be treated as statistically representative of the entire Chagossian population (House of Lords International Relations and Defence Committee, 2025).
The legal position of Chagossians under the treaty should consequently remain distinct from the broader claims advanced by different members and organizations within the community. Questions of historical displacement, political participation, self-determination, compensation, and return overlap with the territorial dispute, but they are not resolved simply by determining which state possesses sovereignty over the archipelago.
9. What Can Britain Legally Do After the Review?
The review leaves several legal paths open. The United Kingdom and Mauritius could still proceed toward entry into force under the existing agreement if the necessary political, diplomatic, and domestic conditions were restored. Because the previous implementing legislation fell at the end of the parliamentary session, a substantially similar settlement would require renewed legislation to make the corresponding changes to UK law.
The parties could instead agree on revisions. Article 16 of the signed text records their agreed mechanism for amendments, providing for changes to be made by written agreement between them (United Kingdom and Mauritius, 2025, art. 16). Because the treaty has not entered into force, that provision should not be treated as an already operative substantive treaty obligation. The underlying point is nevertheless clear: the United Kingdom cannot unilaterally alter a bilateral bargain and make the revised terms binding on Mauritius.
A more extensive change could produce a replacement treaty. That would require agreement between London and Port Louis and the completion of whatever international and domestic procedures applied to the new instrument. The existing treaty would not acquire different terms merely because one signatory wished to revise the settlement.
Britain could also make clear that it no longer intended to become a party to the 2025 agreement. Article 18(a) VCLT provides that the interim obligation to refrain from acts defeating the treaty’s object and purpose continues until a signatory makes such an intention clear. Once that occurs, the Article 18(a) obligation ends prospectively. This would not constitute denunciation or withdrawal from an operative treaty because the Chagos agreement has never entered into force.
That decision would end one route to a negotiated settlement, not the wider international-law dispute. The ICJ advisory opinion, Resolution 73/295, and the 2021 ITLOS judgment would remain unaffected as legal and institutional developments. The United Kingdom would still face the consequences associated with the incomplete decolonization of Mauritius identified by the ICJ.
Also read
Conclusion
The Chagos Islands deal remains a signed treaty that has not entered into force. Its provisions on Mauritian sovereignty, Diego Garcia, payments, security arrangements, and the initial 99-year military regime have not become binding substantive treaty obligations between the parties. The separate interim obligation under Article 18 VCLT arises from signature and should not be confused with partial operation of the 2025 agreement.
International law leaves the United Kingdom room to reconsider its course. It may continue toward entry into force, seek changes agreed with Mauritius, negotiate a replacement settlement, or make clear that it no longer intends to become a party. What it cannot do is unilaterally rewrite the bilateral terms and impose the revised arrangement on Mauritius.
The deeper Chagos dispute does not depend solely on the survival of the 2025 treaty. The ICJ’s conclusions on decolonization, the General Assembly’s response, and the later treatment of Chagos by ITLOS all predate the present review and remain legally relevant. Abandoning the current agreement could end one negotiated route to settlement, but it would not by itself resolve the questions of sovereignty, decolonization, Diego Garcia, or the position of the Chagossian community.
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