International Rulings and the UK–Mauritius Chagos Agreement

On 22 May 2025, following negotiations that began in November 2022 and a joint statement of 3 October 2024 (to learn more, see Sebastian von Massow), the United Kingdom and Mauritius concluded an Agreement, stating that “Mauritius is sovereign over the Chagos Archipelago in its entirety, including Diego Garcia” (Article 1). The preamble explicitly notes that the parties reached the Agreement “having regard to the decisions of international courts and tribunals”.

From a historical perspective, in 1965, the United Kingdom adopted the British Indian Ocean Territory Order to separate the Chagos Archipelago as well as Aldabra, Farquhar, and the Desroches islands from Mauritius and Seychelles for defense purposes. Following the independence of Seychelles in 1976, the islands of Aldabra, Farquhar, and Desroches were restituted to Seychelles. However, the Chagos Archipelago continued to be administered by the United Kingdom in accordance with an exchange of notes that established the United States naval support facility on Diego Garcia. Since then, the United Kingdom and Mauritius disagreed on the sovereignty of the Chagos Archipelago, and the issue has been raised directly or indirectly in several cases without any significant action being taken.

The Chagos Agreement is not only a diplomatic achievement, but also a “contractual transposition” of the decisions of international courts and tribunals. The Agreement acknowledges that the detachment of the Chagos Archipelago and its administration by the United Kingdom constituted unlawful acts (Preamble) and provides reparation to Mauritius.

The decisions of international courts and tribunals

The sovereignty of Mauritius over the Chagos Archipelago has been discussed, at least as a subsidiary issue, in three cases involving the United Kingdom and Mauritius, or in which one of them was a Party or was cited.

Since 1965, the General Assembly has adopted several resolutions, including Resolution A/RES/2066(XX) in which it invited the United Kingdom “to take no action which would dismember the Territory of Mauritius and violate its territorial integrity” (para 4), in accordance with the United Nations Charter and Resolution 1514 (XV) on the independence of colonial countries and peoples. Building upon this, the General Assembly of the United Nations adopted Resolution 71/292 of 22 June 2017, requesting an Advisory Opinion from the International Court of Justice on the Legal consequences of the separation of the Chagos Archipelago from Mauritius in 1965. On 25 February 2019, the Court issued its Advisory Opinion, considering that “the process of decolonization of Mauritius was not lawfully completed when Mauritius became independent in 1968, following the separation of the Chagos Archipelago” (para. 183-3). The Court reaffirmed the erga omnes nature of the right to self-determination and deduced that the administration of the Chagos Archipelago constitutes a continuing internationally wrongful act which engages the international responsibility of the United Kingdom (Advisory Opinion, para 177).  While the General Assembly declared that “the Chagos Archipelago forms an integral part of the territory of Mauritius” (A/RES/73/295, par 1.b) and the UN Geospatial published the new world map (1 February 2020) which incorporated the Chagos Archipelago into Mauritius, the United Kingdom maintained that it “has no doubt about its sovereignty over the Chagos Archipelago” (UK statement, 13 June 2020).

Prior to this case, in 2004, the United Kingdom deposited a list of geographical coordinates declaring a marine protection and preservation zone around the Chagos Archipelago with the Secretary-General of the United Nations pursuant to Article 75(2) of UNCLOS. Mauritius protested against this act on several occasions and ultimately initiated an arbitration procedure against the United Kingdom on 20 December 2010 in accordance with Annex VII of UNCLOS. Essentially, Mauritius claimed that the United Kingdom is not the coastal state, does not have sovereign rights over the Chagos Archipelago, and is not entitled to declare a marine protected area or other maritime zones around the Archipelago (Notice of Arbitration, para 11.2, 11.3; Memorial of Mauritius, paras 1.3, 6.1). The United Kingdom argued that the real dispute (the question of sovereignty) is not subject to the interpretation and application of UNCLOS (Preliminary Objections of the United Kingdom, para 3.10). Although the tribunal agreed with the United Kingdom on this point, it examined the aspects relating to UNCLOS and concluded “unanimously” that the establishment of the Marine Protected Areas surrounding the Chagos Archipelago by the United Kingdom in 2010 violated Articles 2(3), 56(2), and 194(4) of UNCLOS (Award, para. 547-B). In other words, Mauritius is the coastal state and has sovereign rights, jurisdiction, and obligations in the territorial sea and exclusive economic zone around the Chagos Archipelago, in accordance with UNCLOS and the 1965 Fish Stocks Agreement.

Finally, on 28 April 2023, the Special Chamber of the International Tribunal for the Law of the Sea, constituted in the dispute concerning the delimitation of the maritime boundary between Mauritius and the Maldives, delivered its judgment. During the preliminary objection phase, the Maldives invoked the monetary gold principle to contest the jurisdiction of the Special Chamber on the basis of the United Kingdom’s absence as an indispensable party to the maritime boundary dispute (see Xu Qi), drawing an analogy with the East Timor case (Judgment, paras 19, 27, 28). This raised the question of whether Mauritius or the United Kingdom had a coastline facing or adjacent to the Maldives (Preliminary objections, paras. 139, 164, 175). The Special Chamber concluded that the United Kingdom could not have “any legal interests in permanently disposing of maritime zones around the Chagos Archipelago by delimitation” (Preliminary objections, paras. 247, 248). In accordance with the maxim ex injuria jus non oritur, the Special Chamber concluded that the United Kingdom is not the state whose coastline is adjacent to or faces the Maldives. In other words, Mauritius is the coastal state.

The Agreement of 22 May 2025 broadly complies with the rulings of the courts and tribunals. By expressly recognizing the sovereignty of Mauritius over the Chagos Archipelago, including Diego Garcia, and the legal consequences thereof, the United Kingdom is following the logic of completing the process of self-determination, in accordance with the ICJ Advisory Opinion. In fact, as the Special Chamber of the ITLOS constituted in the delimitation of the maritime boundary between Mauritius and Maldives pointed out, “judicial determinations made in advisory opinions carry no less weight and authority than those in judgments because they are made with the same rigour and scrutiny” (Preliminary objections, para 203). Similarly, it takes note of the judgment of the Special Chamber, conceding that it is not the coastal state. Finally, the United Kingdom recognizes that it has no coastal sovereign rights, jurisdiction, and duties in the territorial sea and in the exclusive economic zone, in accordance with the award of the arbitral tribunal under Annex VII to UNCLOS.

The question of reparations

Without prejudice to the Advisory Opinion of the ICJ, which urged that the process of self-determination should be “lawfully completed”, the arbitral tribunal under Annex VII to UNCLOS and the Special Chamber of the ITLOS did not address the issue of reparation. However, as we have seen, their reasoning implies that internationally wrongful acts occurred, for which reparation is required.

In accordance with Article 34 of the ILC Draft Articles on Responsibility of States for Internationally Wrongful Acts, full reparation for damage resulting from an international wrongful act “shall take the form of restitution, compensation and satisfaction, either singly or in combination” (see Dinah Shelton). Restitution requires the responsible state to “wipe out all the consequences of the illegal act and re-establish the situation which would, in all probability, have existed if that act had not been committed” (Factory at Chorzów, p. 47). As for compensation, Article 36(2) of the ILC Draft Articles states that it “shall cover any financially assessable damage including loss of profits insofar as it is established” (see Stephan Wittich). Compensation is ancillary to restitution. It represents an option when restitution is materially impossible or would impose an excessive burden on the responsible state compared to the benefit that the injured state could reasonably expect to obtain through compensation (Article 36 of the ILC Draft Articles). Satisfaction is applied when neither restitution nor compensation is possible, and “may consist in an acknowledgement of the breach, an expression of regret, a formal apology or another appropriate modality” (see Cristina Hoss). The Agreement of 22 May 2025 combines all three forms of reparation.

The United Kingdom expressly returns the Chagos Archipelago, including Diego Garcia, to Mauritius (Article 1). This reflects the definition of restitution in the literature, which states that it may involve “the return of objects, including territory, unlawfully seized, or occupied, as well as of unlawfully detained individuals” (see Attila Tanzi). Furthermore, as compensation, the United Kingdom will pay an annual sum to Mauritius for an initial period of 99 years, which may be extended by a further 40 years (Article 13). The United Kingdom will also establish a trust fund (Article 11) and, where appropriate, offer preference to Mauritian citizens and companies in operating the Diego Garcia base (Article 10). Similarly, the acknowledgement of the “wrongs of the past” in the preamble constitutes a form of reparation in itself (satisfaction). More importantly, the Agreement states that “Mauritius is free to implement a programme of resettlement on the islands of the Chagos Archipelago other than Diego Garcia” (Article 6). It is important to highlight that the resettlement of the Chagossians had been the subject of a public consultation from 4 August 2015 to 27 October 2015 and had been examined by the Foreign Affairs Sub-Committee on the British Overseas Territories before the Agreement of 22 May 2025. On 14 March 2024, the sub-committee urged the United Kingdom to establish a “pilot resettlement programme to provide a right for the Chagossian people to return to their ancestral home” (for more information on the resettlement of the Chagossians in accordance with international law, see Stephen Allen). It is now the responsibility of Mauritius to elaborate a resettlement plan. This is a form of satisfaction as it involves a profound process of remembrance that cannot return the Chagossians to the situation that prevailed before the unlawful act and cannot be objectively assessed in monetary terms. To ensure the Agreement is fully implemented, disputes between the parties may be submitted to a Joint Commission (Article 14, Annex 3) and, if necessary, to an arbitral tribunal (Article 15, Annex 4).

Conclusion

I share the view of Professor Philippe Sands on the Agreement of 22 May 2025, which is that “doing the right thing, for the Chagossians, the environment, national security and the rule of law, can truly be something to be proud of, not to belittle or attack”. Obviously, it would be unrealistic to expect the United Kingdom to grant full reparation to Mauritius and the Chagossians, given the particular context in which they are exercising their right to self-determination. The issue of reparation here involves “large-scale historical wrongs” (For more on this concept, see Carla Ferstman). Nevertheless, the Agreement of 22 May 2025 demonstrates the UK’s good faith and seems to establish the premises for adequate reparation. In short, this unusual example attests to the contribution of ICJ advisory opinions and, more broadly, international case law to the effectiveness of the right to self-determination.