The islands of the Chagos Archipelago were uninhabited until the late 18th century, when the French established copra plantations using slave labour in 1793. The islands have been British territory since 1814 when they were ceded to Britain with Mauritius (which then included the Seychelles). For administrative convenience, and following the French practice, the islands were administered from Mauritius. As for the population of the islands, after emancipation some slaves became contract employees; the population changing over time by import of contract labour from Mauritius and, in the 1950s, from Seychelles, so that by the late 1960s, those living on the islands were contract employees of the copra plantations. Neither they, nor those permitted by the plantation owners to remain, owned land or houses. They had licences to reside there at the discretion of the owners and moved from island to island as work required.

The islands were constituted as the British Indian Ocean Territory in 1965 by an Order in Council under the Royal Prerogative. This comprises all the islands of the Chagos Archipelago and until June 1976 also included the islands of Aldabra, Desroches and Farquhar which were then ceded to the Seychelles, of which they are now part. The Order in Council also provided for the appointment of a Commissioner for the Territory. One function conferred on the Commissioner was the power to make laws for the “peace, order and good government” of the Territory.

The UK paid the colony of Mauritius a £3 million grant in recognition of the detachment of the Chagos Archipelago and amongst other legally binding undertakings, gave a commitment, repeated by successive governments, to cede the islands to Mauritius when no longer required for defence purposes. Similarly, the UK also paid for the construction of an airport in Seychelles in recognition of the detachment of their islands, though these were ceded after Seychelles’ independence.

In a published Exchange of Notes with the US Government in 1966, the Territory was made available to meet the defence needs of the United Kingdom and the United States governments. The Exchange of Notes envisaged US use of Diego Garcia for an initial period of 50 years and could remain in force for a further 20 years beyond 2016 unless either side gives notice to terminate it in the two years before its expiry – that is, from December 2014 to December 2016. There were updates to these Notes in 1972 and 1976.

In 1967 the BIOT Administration acquired the land and the commercial interests of the company operating the copra plantations on the island. As the plantations were in economic decline, their future could not be guaranteed. In 1971, the plantation on Diego Garcia was closed when the island was needed for defence purposes. Closures followed later in the other islands of the Chagos Archipelago. The people affected by these closures were the Mauritian and Seychellois contract workers and their families, who were then given the choice of returning to Mauritius or Seychelles. The majority chose Mauritius where they had close ties and were moved between 1968 and 1973.

In 1971 the Commissioner, acting under instructions from Ministers in London, enacted an Immigration Ordinance which made it unlawful for a person to enter or remain in BIOT without a permit, and allowed those remaining to be removed. This formalised in law the removal of the existing civilian population of the territory to Mauritius or Seychelles and effectively established a prohibition on their return.

Although there are no accurate figures it is believed that between 1400 and 1700 were removed from the islands. In 2002 on the change to British Nationality law many Chagossians who had British Overseas Territories Citizenship automatically, became British Citizens.

Compensation arrangements for the Chagossians were provided by the UK in the 1970s (£650,000 to Mauritius for resettlement) and the 1980s (a further £4m to Mauritius and representatives of the Chagossians in the form of a trust fund).

There is a small community of Chagossians based in Crawley, Sussex, and another in Manchester. There are also communities of former islanders and their descendants remaining in Seychelles and Mauritius. The British Indian Ocean Territory Administration has facilitated several (roughly annual) visits to the Territory by the eldest Chagossians, and a programme of environmental training for UK-based Chagossians that allows some to become involved in scientific work in the Territory.