Sovereignty over the Chagos Archipelago has been disputed between Mauritius and the United Kingdom, which administers it as the British Indian Ocean Territory (BIOT). Mauritius has repeatedly claimed the Chagos Archipelago as part of its territory and that the British claim is a violation of United Nations resolutions banning the dismemberment of colonial territories before independence. Given the absence of any progress with the UK, Mauritius took up the matter in various legal and political fora.

Between 1967 and 1973, the entire population of the Chagos Archipelago was either prevented from returning or forcibly removed by the United Kingdom. The removals were carried out to make way for the establishment of a joint UK–United States military base on Diego Garcia. The main forcible removal of Diego Garcia's population took place in July and September 1971. On 18 March 2015, the Permanent Court of Arbitration unanimously held that the marine protected area (MPA) which the United Kingdom declared around the Chagos Archipelago in April 2010 was created in violation of international law. The UK had argued that those undertakings were not binding and had no status in international law.

On 22 June 2017, by a margin of 94 to 15 countries, the UN General Assembly asked the International Court of Justice (ICJ) to give an advisory opinion on the separation of the Chagos Archipelago from Mauritius before the country's independence in the 1960s. In September 2018, the International Court of Justice began hearings on the case. 17 countries argued in favour of Mauritius. The UK and its allies argued that this matter should not be decided by the court but should be resolved through bilateral negotiations. On 25 February 2019, the judges of the International Court of Justice by thirteen votes to one stated that the United Kingdom is under an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible. Only the American judge, Joan Donoghue, voted in favor of the UK. The president of the court, Abdulqawi Ahmed Yusuf, said the detachment of the Chagos Archipelago in 1965 from Mauritius had not been based on a "free and genuine expression of the people concerned." "This continued administration constitutes a wrongful act," he said, adding "The UK has an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible and that all member states must co-operate with the United Nations to complete the decolonization of Mauritius."

On 3 November 2022, it was announced that the United Kingdom and Mauritius had decided to begin negotiations on sovereignty over the BIOT, taking into account the recent international legal proceedings. On 3 October 2024, the UK agreed to hand over the Chagos Islands to Mauritius, although this was controversial given the presence of a U.S. military base on one of the islands. The United States had endorsed the agreement though months later, US president Donald Trump reversed this position and called the deal as an "act of great stupidity" and was against the deal. In geopolitical context, this settlement is expected to benefit India which is already maintaining a high presence in the area in close cooperation with Mauritius, while India is undertaking military cooperation with the U.S. as it finds ways to deal with the rising footprints of China in the Indian Ocean. On 22 May 2025, the agreement was signed by the UK and Mauritius; the dispute will end once the deal is ratified by both parties. Opponents of the deal have cited Article 298 (b) of the United Nations Convention on the Law of the Sea (UNCLOS) as giving the British government an invokable legal exemption from "disputes concerning military activities, including military activities by government vessels and aircraft engaged in non-commercial service", and have argued that the British government's case was misleading. In April 2026, it was announced that the deal would not be approved before the end of the parliamentary session, following Trump's criticism of the UK and the deal.

History The Constitution of Mauritius states that the Republic of Mauritius includes the islands of Mauritius, Rodrigues, Agaléga, Cargados Carajos and the Chagos Archipelago, including Diego Garcia and any other island comprised in the State of Mauritius. The Government of the Republic of Mauritius has stated that it does not recognise the British Indian Ocean Territory (BIOT) which the United Kingdom created by excising the Chagos Archipelago from the territory of Mauritius prior to its independence, and claims that the Chagos Archipelago including Diego Garcia forms an integral part of the territory of Mauritius under both Mauritian law and international law.

In 1965, the United Kingdom split the Chagos Archipelago away from Mauritius, and the islands of Aldabra, Farquhar, and Desroches from the Seychelles, to form the British Indian Ocean Territory. The islands were formally established as an overseas territory of the United Kingdom on 8 November 1965. However, with effect from 23 June 1976, Aldabra, Farquhar, and Desroches were returned to the Seychelles on their attaining independence.

In 2012, the African Union and the Non-Aligned Movement have expressed unanimous support for Mauritius. On 18 March 2015, the Permanent Court of Arbitration unanimously held that the marine protected area (MPA) which the United Kingdom declared around the Chagos Archipelago in April 2010 was created in violation of international law. The Prime Minister of Mauritius has stated that this is the first time that the country's conduct with regard to the Chagos Archipelago has been considered and condemned by any international court or tribunal. He described the ruling as an important milestone in the relentless struggle, at the political, diplomatic, and other levels, of successive Governments over the years for the effective exercise by Mauritius of the sovereignty it claims over the Chagos Archipelago. The tribunal considered in detail the undertakings given by the United Kingdom to the Mauritian Ministers at the Lancaster House talks in September 1965. The UK had argued that those undertakings were not binding and had no status in international law. The Tribunal firmly rejected that argument, holding that those undertakings became a binding international agreement upon the independence of Mauritius, and have bound the UK ever since. It found that the UK's commitments towards Mauritius in relation to fishing rights and oil and mineral rights in the Chagos Archipelago are legally binding.

Historical background Beginning in the late 15th century, Portuguese explorers began to venture into the Indian Ocean and recorded the location of Mauritius and the other Mascarene Islands, Rodrigues and Réunion (the latter presently a French overseas department). In the 16th century, the Portuguese were joined by Dutch and English sailors, both nations having established East India Companies to exploit the commercial opportunities of the Indian Ocean and the Far East. Although Mauritius was used as a stopping point in the long voyages to and from the Indian Ocean, no attempt was made to establish a permanent settlement.

The first permanent colony in Mauritius was established by the Dutch East India Company in 1638. The Dutch maintained a small presence on Mauritius, with a brief interruption, until 1710 at which point the Dutch East India Company abandoned the island. Following the Dutch departure, the French government took possession of Mauritius in 1715, renaming it the Île de France.

The Chagos Archipelago was known during this period, appearing on Portuguese charts as early as 1538, but remained largely untouched. France progressively claimed and surveyed the Archipelago in the mid-18th century and granted concessions for the establishment of coconut plantations, leading to permanent settlement. Throughout this period, France administered the Chagos Archipelago as a dependency of the Ile de France.

In 1810, the British took control of the Ile de France during the Napoleonic Wars and renamed it Mauritius. France signed the Treaty of Paris in 1814, formally relinquishing control to the United Kingdom of the territories that comprised the British Colony of Mauritius, which consisted of the main island of Mauritius along with its dependencies Rodrigues, Agaléga, St. Brandon, Tromelin (disputed) and the Chagos Archipelago.

British administration of Mauritius and the Chagos Archipelago

From the date of the cession by France until 8 November 1965, when the Chagos Archipelago was detached from the colony of Mauritius, the Archipelago was administered by the United Kingdom as a Dependency of Mauritius. During this period, the economy of the Chagos Archipelago was primarily driven by the coconut plantations and the export of copra (dried coconut flesh) for the production of oil, although other activities developed as the population of the Archipelago expanded. British administration over the Chagos Archipelago was exercised by various means, including by visits to the Chagos Archipelago made by Special Commissioners and Magistrates from Mauritius.

Although the broad outlines of British Administration of the colony during this period are not in dispute, the Parties disagree as to the extent of economic activity in the Chagos Archipelago and its significance for Mauritius, and on the significance of the Archipelago's status as a dependency. Mauritius contends that there were "close economic, cultural and social links between Mauritius and the Chagos Archipelago" and that "the administration of the Chagos Archipelago as a constituent part of Mauritius continued without interruption throughout that period of British rule". The United Kingdom, in contrast, submits that the Chagos Archipelago was only "very loosely administered from Mauritius" and "in law and in fact quite distinct from the Island of Mauritius." The United Kingdom further contends that "the islands had no economic relevance to Mauritius, other than as a supplier of coconut oil" and that, in any event, economic, social and cultural ties between the Chagos Archipelago and Mauritius during this period are irrelevant to the Archipelago's legal status.

Independence of Mauritius

Beginning in 1831, the administration of the British Governor of Mauritius was supplemented by the introduction of a Council of Government, originally composed of ex-officio members and members nominated by the Governor. The composition of this council was subsequently democratized through the progressive introduction of elected members. In 1947, the adoption of a new Constitution for Mauritius replaced the Council of Government with separate Legislative and Executive Councils. The Legislative Council was composed of the Governor as president, 19 elected members, 12 members nominated by the Governor and 3 ex-officio members.

The first election of the Legislative Council took place in 1948, and the Mauritius Labour Party (the "MLP") secured 12 of the 19 seats available for elected members. The MLP strengthened its position in the 1953 election by securing 14 of the available seats, although the MLP lacked an overall majority in the Legislative Council because of the presence of a number of members appointed by the Governor.

The 1953 election marked the beginning of Mauritius’ move towards independence. Following that election, Mauritian representatives began to press the British Government for universal suffrage, a ministerial system of government and greater elected representation in the Legislative Council. By 1959, the MLP-led government had openly adopted the goal of complete independence.