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 60 square kilometres (23 square miles). The largest and most southerly island is Diego Garcia, 27 square kilometres (10 square miles), 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.
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.
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 3,000 hectares (7,400 acres).
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".