Geography and Administration
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, which covers 27 square kilometres (10 square miles) and is 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.
Territorial Dispute
Mauritius claims that the British government separated the Chagos Archipelago from Mauritius in 1965, creating a new colony called the British Indian Ocean Territory (BIOT). The United Kingdom disputes this, insisting that the Chagos Islands had no historical or cultural ties to Mauritius and were only governed during the colonial period from Mauritius as an administrative convenience.
Since the 1980s, the Government of Mauritius has sought to gain control over the Chagos Archipelago. A 2019 advisory opinion of the International Court of Justice called for the islands to be given to Mauritius, a position subsequently supported by both the United Nations General Assembly and the International Tribunal for the Law of the Sea.
Negotiations between the UK and Mauritius began in 2022, culminating in a 2024 understanding that the UK would cede the territory to Mauritius while retaining the joint US-UK military base on 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. However, following backlash from United States President Donald Trump in 2026, ratification has been suspended.
Population and Military Use
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, when approximately 2,000 people were expelled by the British government to Mauritius and Seychelles. 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. 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.
Early History
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. In 1810, Mauritius was captured by the United Kingdom, and France subsequently ceded the territory in the 1814 Treaty of Paris.
Settlement and Development
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.
An airstrip was built on Diego Garcia during the Second World War, along with other military facilities. In 1962 the plantations were bought by Chagos-Agalega Company, based in the Seychelles, and at this time they covered 3,000 hectares (7,400 acres).
Formation of the Territory
Beginning in February 1964, the United Kingdom and the United States began secret talks about establishing 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. In 1965, the United Kingdom split the Chagos Archipelago from Mauritius to form the British Indian Ocean Territory (BIOT). The islands were formally established as an overseas territory of the United Kingdom on 8 November 1965.
Mauritius was eventually paid £3 million to compensate for the territory's separation, with an additional £650,000 paid in 1972 for relocation costs. However, the United Nations General Assembly passed Resolution 2066 on 16 December 1965, stating that this detachment of part of the colonial territory of Mauritius was against customary international law.