The British Indian Ocean Territory (BIOT) is a British Overseas Territory situated in the Indian Ocean. The territory comprises the seven atolls of the Chagos Archipelago with over 1,000 individual islands, many very small, amounting to a total land area of . The largest and most southerly island is Diego Garcia, , the site of a Joint Military Facility of the United Kingdom and the United States. Official administration is remote from London, though the local capital is often regarded as being on Diego Garcia.
Mauritius claims that the British government separated the Chagos Archipelago from Mauritius, creating a new colony, the British Indian Ocean Territory (BIOT). There are ongoing negotiations on this dispute.
Since 1971, only personnel of the British and United States military and associated contractors have been legally allowed to live on the Chagos Islands. The forced removal of Chagossians from the Chagos Archipelago occurred between 1968 and 1973. The Chagossians, then numbering about 2,000 people, were expelled by the British government to Mauritius and Seychelles, even from the outlying islands far away from the military base on Diego Garcia. Despite calls from numerous human rights organisations, the British government has repeatedly denied Chagossians the right of return. In 2026, six Chagossians returned to Île du Coin in an attempt to reestablish a permanent settlement, without seeking government permission; their effort is being challenged in court. In March 2026, the BIOT Supreme Court ruled that the Chagossians had the right to live on the island, but the territorial administration is appealing the ruling.
Since the 1980s, the Government of Mauritius has sought to gain control over the Chagos Archipelago, which was separated from the then Crown Colony of Mauritius by the UK in 1965 to form the British Indian Ocean Territory. A 2019 advisory opinion of the International Court of Justice called for the islands to be given to Mauritius. Both the United Nations General Assembly and the International Tribunal for the Law of the Sea subsequently reached similar decisions. Negotiations between the UK and Mauritius began in 2022, and culminated in a 2024 understanding that the UK would cede the territory to Mauritius for possible resettlement while retaining the joint US-UK military base on Diego Garcia. While the ICJ and UN have supported decolonization of the islands, the UN Committee on the Elimination of Racial Discrimination has expressed "deep concern" at the terms of the deal, indicating that it does not go far enough to allow Chagossian people to exercise their cultural rights and preserve their cultural heritage, and that it forbids Chagossian settlements on the island of Diego Garcia.
A treaty was signed on 22 May 2025 that would formally transfer sovereignty of the territory to Mauritius once ratified, with the Diego Garcia military base remaining under British control during a 99-year lease. Following backlash from United States President Donald Trump in 2026, ratification has been suspended.
History Maldivian mariners knew of the Chagos Islands, which were known as Fōlhavahi in the northern Maldives and Hollhavai in the southern Maldives. Hollhavai also referred to other islands scattered throughout the Indian Ocean, such as the Seychelles. According to Southern Maldivian oral tradition, traders and fishermen were occasionally lost at sea and got stranded on one of the islands of the Chagos. Eventually they were rescued and brought back home. The coconut crabs of the islands may have inspired Maldivian folklore about giant hermit crabs. These islands were judged to be too far away from the seat of the Maldivian crown to be settled permanently. Thus for many centuries the Chagos were ignored.
Early settlement
The islands of the Chagos Archipelago were charted by Vasco da Gama in the early 16th century, and then claimed in the 18th century by France as a possession of Mauritius. They were first settled in the 18th century by African slaves and Indian contractors brought by Franco-Mauritians to found coconut plantations. At some point Diego Garcia hosted a leper colony for patients from Mauritius, who were treated with turtle oil. In 1810, Mauritius was captured by the United Kingdom, and France subsequently ceded the territory in the 1814 Treaty of Paris.
The United Kingdom abolished slavery in 1833, although those on what would become the BIOT continued work as indentured labourers. In 1883 the plantations were bought by the Société Huilière de Diego et Peros. As of 1900, there were 426 families on the islands, 60 per cent descended from the original African-Malagasy slaves, and 40 per cent from South Asians brought over as indentured labour. Chagossian creole developed as a common language. The lives of the inhabitants remained effectively controlled by the company, a system which persisted until the 1960s.
An airstrip was built on Diego Garcia during the Second World War, along with other military facilities. The military left once the war ended. In 1962 the plantations were bought by Chagos-Agalega Company, based in the Seychelles. At this time they covered .
Formation The descendants of those brought to the islands to farm coconuts had developed into the Chagossian people. There are no official population figures, but an estimate from 1968 found 866 inhabitants: 336 in Diego Garcia, 162 in the Salomon islands, and 168 in Peros Banhos. The vital statistics were likely similar to other Indian Ocean islands. The birth rate on the Chagos archipelago was probably around 48 in every 1000. The birth rate for the former Seychellois islands was nominally lower, likely due to inhabitants more easily able to travel to the Seychelles for medical needs. Mortality before the age of 1 was around 98 per 1,000 live births. The overall death rate probably averaged under 12 per 1,000 people. Housing was simple, mostly using local wood and coconut leaves, with some imported concrete and corrugated iron. Most water was captured from rain, although there were groundwater wells. The islands at this time produced not only copra from the plantations, but fish, and fruit in the case of the Chagos archipelago. Alcohol was produced for local consumption, from coconut, sugar cane, and other materials. It is likely there was some malnutrition.
Beginning in February 1964, the United Kingdom and the United States began secret talks about a military base in Diego Garcia. The United States wanted a territory with no local population, and the islands were lightly inhabited compared to other potential locations. The initial option of Aldabra was dropped due to international interest in its ecosystem. In 1965, the United Kingdom split the Chagos Archipelago from Mauritius and the islands of Aldabra, Farquhar, and Desroches (Des Roches) from the Seychelles to form the British Indian Ocean Territory (BIOT). The purpose was to allow the construction of military facilities for the mutual benefit of the United Kingdom and the United States. The islands were formally established as an overseas territory of the United Kingdom on 8 November 1965. While it initially sought for the base to be given on a lease, Mauritius was eventually paid £3 million to compensate for the territory's separation, under pressure due to then-ongoing independence negotiations. An additional £650,000 was paid in 1972 for relocation costs.
A few weeks after the decision to detach the Chagos Archipelago from Mauritius, the United Nations General Assembly passed Resolution 2066 on 16 December 1965, which stated that this detachment of part of the colonial territory of Mauritius was against customary international law as recorded earlier in the Declaration on the Granting of Independence to Colonial Countries and Peoples of 14 December 1960. This stated that "Any attempt aimed at the partial or total disruption of the national unity and the territorial integrity of a country is incompatible with the purposes and principles of the Charter of the United Nations".
Mauritius became an independent Commonwealth realm in March 1968, and subsequently became a republic, also within the Commonwealth, in March 1992.
Expulsion of the Chagossians
In April 1967, the British government purchased the privately owned copra plantations for £600,000 and closed them. Over the next five years, the British authorities removed the entire population of about 2,000 people, known as Chagossians (or Ilois), from Diego Garcia and two other Chagos atolls, Peros Banhos and Salomon Islands, to Mauritius and the Seychelles. The expulsion was carried out by preventing Chagossians who had left from returning from 1967, assisted by the Moulinie and Company (Seychelles), Limited company which the UK had set up to run the plantations. In 1968, when more workers were needed, non-Chagossian temporary workers were allowed in from the Seychelles. The remaining population was removed from Diego Garcia in January 1971, to meet a July 1971 deadline requested by the US. The populations of Peros Banhos and the Salomon Islands were removed starting from June 1972. The population as of 1965 was around 1,360 Chagossians, plus temporary workers from the Seychelles. Another few hundred Chagossians already lived outside of the islands. The UK informed the United Nations that BIOT had no indigenous population, meaning the UK would not have to send reports on the territory to the UN.
In 1971, the United Kingdom and the United States signed a treaty, leasing the island of Diego Garcia to the US military for the purposes of building a large air and naval base on the island. The deal was important to the British government, as the United States granted it a substantial discount on the purchase of Polaris nuclear missiles in return for the use of the islands as a base. The island was strategically located near oil shipping routes, and Iran. It would counter any Soviet threat in the region.
Development
As of 1971, the population of Aldabra, Farquhar, and Desroches was 246. This dropped to 155 in 1975, likely reflecting uncertainty about these islands' future status. On 23 June 1976, these islands were returned to the Seychelles, which became independent as a republic on 29 June 1976; the islands now form part of the Outer Islands district of the Seychelles. Subsequently, the territory has consisted only of the six main island groups comprising the Chagos Archipelago.
Work on the military base commenced in 1971, with a large airbase with several long range runways constructed, as well as a harbour suitable for large naval vessels. Although classed as a joint UK/US base, in practice it is primarily staffed by the US military, although the UK maintains a garrison at all times, and Royal Air Force (RAF) long-range patrol aircraft are deployed there.
During the 1980s, Mauritius asserted a claim to sovereignty for the territory, citing the 1965 separation as illegal under international law, despite their apparent agreement at the time. The UK did not agree with the legal argument, but agreed to cede the territory to Mauritius when it was no longer required for defence purposes. According to the CIA World Factbook, the Seychelles also had a sovereignty claim on the islands. In 1982, the UK and US agreed waste would be shipped away from the territory. On 27 December 1984, Mauritius established an exclusive economic zone (EEZ) which included the waters around the BIOT, an action protested by the UK.