July 2025
Summary
The 2025 UK–Mauritius Agreement on the Chagos Archipelago presents a complex case of state succession, wherein sovereignty is formally transferred to Mauritius while the United Kingdom retains extensive control over Diego Garcia for military purposes. Although this territorial transfer does not alter the legal personality of either state, it has significant implications for their respective rights and obligations – particularly in areas such as representation within international organisations, participation in decision-making processes, and the scope of treaty and financial commitments. This arrangement may give rise to several problems, including incomplete treaty succession, disruptions to the functioning of international organisations, and legal uncertainty for transnational businesses. To mitigate these risks, governments need to clarify treaty obligations, international organisations have to coordinate the transition of membership and responsibilities, and businesses should assess their compliance and risk strategies in light of evolving governance frameworks and overlapping jurisdictions.
General observation
On 22 May 2025 the United Kingdom and Mauritius concluded An Intergovernmental Agreement concerning the Chagos Archipelago including Diego Garcia,[1] which marks a significant development in the law and practice of state succession. From the perspective of the State Succession Index, this treaty provides a unique example of a negotiated transfer of sovereignty with ongoing foreign military presence, creating important legal and operational consequences for relevant governments, international organisations, and transnational businesses.
According to Article 1 of the Agreement, “Mauritius is sovereign over the Chagos Archipelago in its entirety, including Diego Garcia.” However, neither the Agreement nor the accompanying Explanatory Memorandum provides a legal explanation of how Mauritius acquired sovereignty over the Archipelago. Instead, Article 1 merely asserts Mauritian sovereignty as an accepted fact, without elaborating on the legal basis or process through which that sovereignty was recognised or transferred.
In para 3(1) of the Explanatory Memorandum on the Agreement[2] the Parties agreed that “On 8 November 1965, the UK created BIOT by detaching a number of islands from the colony of Mauritius and the colony of the Seychelles. At the time of detachment, the UK committed that the islands detached from the colony of Mauritius would be ceded to Mauritius when they were no longer needed for defence purposes.” By including this historical reference, the Parties appear to indirectly acknowledge that the UK is now ceding the Chagos Archipelago to Mauritius – a formulation that reflects the legal concept of cession, understood in international law as the peaceful transfer of territory from one state to another. This understanding aligns with the definition of state succession under Article 2(1)(b) of the 1978 Vienna Convention on Succession of States in Respect of Treaties, which describes succession as “the replacement of one State by another in the responsibility for the international relations of territory.” [3] Although neither the UK nor Mauritius is a party to this Convention, specific provisions of the treaty may be applicable through customary international law.
This transfer of territory does not alter the legal personality of either the United Kingdom or Mauritius, but it does carry significant implications for their respective rights and obligations. These include changes in their status and participation within international organisations, influence over decision-making processes, and the scope and application of treaty and financial obligations. One of the most critical areas of impact concerns the functioning and governance of international organisations, where the UK’s withdrawal in respect of the Chagos Archipelago and Mauritius’s assumption of new responsibilities may affect voting rights, budgetary contributions, and institutional representation. Such transitions may create legal and procedural challenges that require careful coordination to avoid disruption.
Core State Succession Issues
1. Transfer of Sovereignty vs Retained Control
As mentioned under Article 1 of the Agreement, Mauritius is recognised as the sovereign over the Chagos Archipelago. However, under Article 2, the UK is authorised to exercise all rights and authorities necessary for the operation of the US-UK base on Diego Garcia for a minimum of 99 years. This reflects de-facto partial transfer of effective control, where sovereignty and administration are decoupled—a configuration that risks long-term legal ambiguity on specific issues in respect of Diego Garcia.
2. Succession to Treaties and Agreements
The legal regime governing State succession to Treaties in the form of a cession has traditionally been considered to be settled.[4] It is addressed in Article 15 of the Vienna Convention, which reflects the idea commonly referred to as the “moving treaty frontiers” rule: The treaties of the predecessor State cease to be in force in respect of the territory to which the succession of States relates from the date of the succession of States. Article 29 of the Vienna Convention on the Law of Treaties 1969 to which both States are Parties, assumes that when a State concludes a treaty, that treaty applies to that State’s entire territory, strengthens Article 15. It is generally accepted that Article 15 reflects customary international law.[5] Accordingly, upon the entry into force of the 2025 Agreement, the United Kingdom’s treaty obligations will cease to apply to the Chagos Archipelago, and Mauritius’s treaties will, in principle, begin to apply to the territory—subject to specific exceptions concerning Diego Garcia, where the UK retains certain functional authorities under the Agreement.
This change in sovereignty may give rise to legal and operational challenges regarding the applicability of bilateral and multilateral treaties to which the UK and Mauritius are parties—particularly in relation to territorial scope, financial obligations, and participation in decision-making processes. If unaddressed before the Agreement enters into force, these issues could cause confusion or disruption. To avoid such outcomes, governments with treaty relations involving the Chagos Archipelago should proactively reassess their legal obligations and consider how the territorial enlargement of Mauritius may affect their treaty relations.
The Agreement stipulates that existing international obligations shall not be affected except as expressly stated (Art. 7). However, the overlapping jurisdictions on defence, security, environment, and migration necessitate complex bilateral and tripartite arrangements (including with the US), raising questions about treaty continuity and potential disputes. The Parties must clearly limit authorities and create mechanisms of quick resolution of uncertainties and disagreements.
3. Succession to International Organisations
The treaty addresses transitions in international legal personality in certain organisations. The UK agrees to downgrade or withdraw from positions in relevant international bodies (e.g., IOTC, UPU), while Mauritius is recognised as the new sovereign. Still, arrangements for Diego Garcia’s military-related representation in organisations such as the International Telecommunication Union and the Ramsar Convention remain exceptional and fragmented.
The UK agree to ensure its membership in international organisations is consistent with Mauritian sovereignty over the Chagos Archipelago in Article 1 (Article 8). This obligation is further elaborated in a separate exchange of letters dated the same date as the treaty, in which the UK agrees that its membership of the following international organisations in respect of the Chagos Archipelago shall transition to reflect Article 1: UN Specialized Agencies / UN Bodies, North Indian Ocean Hydrographic Commission, Indian Ocean Naval Symposium, Indian Ocean Tuna Commission and Central Asian Flyway. The Parties further agree to implement bespoke arrangements with the following international organisations in respect of Diego Garcia: The International Telecommunication Union, The Comprehensive Nuclear-Test-Ban Treaty Organisation, and the Convention on Wetlands of International Importance especially as Waterfowl Habitat (known as the Ramsar Convention).
The Parties further agree that none of their existing international obligations or arrangements now in force or effect between them and any third party is in conflict with the provisions of the treaty, and that nothing in the treaty shall affect the status of existing international obligations or arrangements except as expressly provided for in the treaty (Article 7(1)(a)). The treaty thereby ensures compatibility of the treaty with other international obligations of the Parties, current and future.
Nevertheless, the mentioned organisations should reassess how the change in membership could affect their operations, decision-making processes, and financial obligations.
4. Succession to International Responsibility
Under the Agreement, Mauritius is set to assume international responsibility for the territory upon the Agreement’s entry into force. This includes obligations under international law relating to environmental protection, maritime governance, and regulatory oversight. However, due to the continued UK–US control over Diego Garcia, responsibility for military activities and associated compliance with international humanitarian and human rights law remains with the UK. In areas governed jointly—such as migration control and maritime security—international responsibility may be shared or subject to further clarification through the Joint Commission.
The UK also retains liability for acts committed before the Agreement takes effect, including legacy issues involving the displacement of Chagossians. The preambular section of the Agreement acknowledges “the wrongs of the past” and recognises that “the past treatment of Chagossians has left a deeply regrettable legacy,” with both Parties expressing a shared commitment to support the welfare of all Chagossians. Under Article 6, Mauritius is authorised to exercise its sovereignty to implement a programme of resettlement on the islands of the Chagos Archipelago, excluding Diego Garcia, in accordance with the Agreement and Mauritian law. Additionally, Article 11(1)(b) provides that the UK will capitalise a Trust Fund established by Mauritius for the benefit of Chagossians, with an initial contribution of GBP 40 million. Article 11(3) further states that the Agreement constitutes “the full and final settlement of all claims by Mauritius in relation to the Chagos Archipelago.” While Mauritius has agreed to settle all inter-state claims with the UK, this does not exclude possible claims by individuals or third parties under international law.
5. Legal Certainty and Private Rights
The legal system of Mauritius is a hybrid system, combining elements of civil law (influenced by French law) and common law (influenced by British law). Upon the entry into force of the Agreement, this hybrid legal system will, in principle, become applicable to the Chagos Archipelago. Given that the UK previously applied common law in the territory, the transition to Mauritius’s mixed legal system is unlikely to pose significant challenges. The compatibility between the two systems, particularly in areas influenced by common law, should facilitate a smooth legal transition. This process is further simplified by the absence of a permanent resident population in the Chagos Islands.
At the same time, Mauritius retains regulatory jurisdiction over areas such as environmental protection, natural resources, and commercial activities unrelated to the base—but often subject to Joint Commission approval (Annex 3 to the Agreement). Businesses entering and operating in the region must navigate dual authority, triggering compliance and licensing uncertainties.
Entry into force and term of the treaty
The duration of the treaty is 99 years from entry into force (Article 13(1) of the Agreement), extendable with the agreement of both Parties (Articles 13(2)-(3)). Article 18 of the Agreement provides that the treaty shall enter into force on the first day of the first month following the date of receipt of the last notification by the Parties of completion of their respective internal requirements and procedures. According to para 4 of the Explanatory Memorandum Mauritian procedures will depend on Mauritian Ministerial decisions, but will likely take no more than 6 months. From the UK perspective the treaty will only come into force when the necessary UK primary and secondary legislation is in place.
The United Kingdom is already taking steps to implement the Agreement through domestic legislation. Notably, on 15 July 2025, the UK Government introduced the Diego Garcia Military Base and British Indian Ocean Territory Bill in Parliament, which includes a provision to amend the British Nationality Act 1981 to reflect that the British Indian Ocean Territory (i.e. the Chagos Archipelago) will no longer be a British Overseas Territory. Given the complexity of the Agreement and the scope of legislative changes required on both the UK and Mauritian sides, it is reasonable to presume that the Agreement may enter into force by the end of 2025 or early 2026. The Parties may also agree internally to select a fixed and practical date for entry into force – such as 1 January 2026 – which would allow international organisations to align budget cycles, adjust reporting systems, and implement updated decision-making procedures at the start of the calendar year.
Thus, with approximately half a year remaining until the likely entry into force of the Agreement, international actors have a valuable window of time to assess and adjust their policies and legal frameworks. This period should be used to ensure a smooth legal and institutional transition, allowing for the effective implementation of the new arrangements from the beginning of 2026.
Recommendations to international actors
The international community should support the implementation of the 2025 UK–Mauritius Agreement on the Chagos Archipelago as a concrete step toward upholding the international legal order and reinforcing the rule of law. In line with the 2019 International Court of Justice’s Advisory Opinion[6] and UN General Assembly Resolution 73/295, states and international organisations should recognise Mauritian sovereignty over the Archipelago, ensure consistency in institutional practice, and align policies with principles of self-determination and territorial integrity. Coordinated support—including legal, technical, and capacity-building assistance—will promote peaceful implementation, affirm the authority of international courts, and strengthen global adherence to the rule of law.
For Governments, it is important to clarify treaty succession obligations through appropriate depositary notifications and legal declarations, particularly concerning treaties applicable to the Chagos Archipelago. Governments should also support Mauritius in establishing and transitioning its domestic legal frameworks to administer its newly recognised sovereign rights, while ensuring legal certainty for international investors and bilateral partners. Governments should also ensure that any past or ongoing legal obligations connected to the Chagos Archipelago are clearly identified and addressed in light of the transfer of sovereignty.
For International Organisations, close coordination with both Mauritius and the United Kingdom is essential to manage a phased and orderly transition in representation, treaty participation, financial and reporting responsibilities, and decision-making processes, all of which must reflect the shift in sovereignty. Organisations should also consider offering neutral capacity-building support to Mauritian institutions, especially in governance areas previously managed by the UK, such as fisheries oversight and maritime security. International organisations should also monitor the implementation of provisions related to the welfare of the Chagossians and the establishment of the GBP 40 million Trust Fund.
For Transnational Businesses, it is important that businesses are encouraged to proactively engage with both UK and Mauritian authorities to clarify jurisdictional issues, including taxation, contract enforcement, and dispute resolution, to maintain operational certainty and legal coherence.
Prepared by Mirbek Sydygaliev, with research assistance from Aiperi Asekova.
[1] https://assets.publishing.service.gov.uk/media/682f25afc054883884bff42a/CS_Mauritius_1.2025_Agreement_Chagos_Diego_Garcia.pdf
[2] https://www.gov.uk/government/publications/ukmauritius-agreement-concerning-the-chagos-archipelago-including-diego-garcia-cs-mauritius-no12025
[3] https://legal.un.org/ilc/texts/instruments/english/conventions/3_2_1978.pdf
[4] James G. Devaney, Christian J. Tams. Succession in respect of cession, unification and separation of States, in General International Law in International Investment Law: A Commentary, 2024. Oxford. P.437.
[5] James G. Devaney, Christian J. Tams. P.437.