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.

[18]: 65  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).

[19] 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.

)[18]: 62 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.

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[23] and the Seychelles.

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.

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,[year needed] 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.

[28]: 49  On 27 December 1984, Mauritius established an exclusive economic zone (EEZ) which included the waters around the BIOT, an action protested by the UK.

This flag, which also contains the Union Jack, has depictions of the Indian Ocean, where the islands are located, in the form of white and blue wavy lines and also a palm tree rising above the British crown.

[28]: 53 The islanders, who now mainly reside in Mauritius and Seychelles, have continually asserted their right to return to Diego Garcia, winning important legal victories in the High Court of England and Wales in 2000, 2006, and 2007.

[33] On 22 October 2008, the British government won an appeal to the House of Lords regarding the royal prerogative used to continue excluding the Chagossians from their homeland.

[41] In its advisory opinion, the Court concluded that "the process of decolonisation of Mauritius was not lawfully completed when that country acceded to independence", and that "the United Kingdom is under an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible".

[42][43] On 22 May 2019, the UNGA adopted a resolution citing the ICJ advisory opinion, affirming that "the Chagos Archipelago forms an integral part of the territory of Mauritius".

[50][51] The main purpose of the 15-day Mauritian expedition is to survey the unclaimed Blenheim Reef, to discover for a forthcoming International Tribunal for the Law of the Sea hearing if it is exposed at high tide so is claimable.

[17][55] In 2022, a dispute arising from American defence contractor KBR allegedly prohibiting holidays during a period of low flight demand during the COVID-19 pandemic led to 800 Overseas Filipino Workers being unable to leave Diego Garcia.

[60] On 3 November 2022, Patrick Wintour of the Guardian reported that the UK and Mauritius had begun negotiations on sovereignty over the British Indian Ocean Territory, taking into account the international legal proceedings.

[65] In November 2024 the Maldives staked a surprise claim to the Chagos Islands, challenging ongoing negotiations between the UK and Mauritius over the strategically vital territory.

The first exchange of notes, signed on 30 December 1966, constituted an agreement concerning the availability for defence purposes of the British Indian Ocean Territory.

The largest island is Diego Garcia, which at 32.5 square kilometres (12.5 sq mi) accounts for about half of the territory's total land area.

[88] Yacht crews seeking safe passage across the Indian Ocean may apply for a mooring permit for the uninhabited Outer Islands (beyond Diego Garcia),[89] but must not approach within 3 nautical miles (5.6 kilometres; 3.5 miles), land on, or anchor at islands designated as Strict Nature Reserves, or the nature reserve within the Peros Banhos atoll.

A rise in illegal fishing in the early 2020s led to a decrease in sharks, and the Royal Navy ship HMS Tamar was sent to assist with monitoring efforts.

In October 2021, 89 Sri Lankan Tamils, including 20 children, who were traveling from India to Canada in a vessel which ran into distress, were intercepted and escorted to Diego Garcia by the British military.

Ten group members challenged this decision in the BIOT Supreme Court, on the basis that the decision-making process was flawed, and were granted a judicial review that was due to be heard in September 2023.

The solicitor for eight of the group said: "Our clients are relieved that the BIOT commissioner has finally agreed to withdraw the unlawful decisions to forcibly return them to Sri Lanka where they face risk of torture and persecution.

[106] The Navy Morale, Welfare and Recreation (MWR) section provides several facilities on Diego Garcia, including a library, outdoor cinema, shops, and sports centres, with prices in US dollars.

An amateur club station, VQ9X, was sponsored by the US Navy for use by operators both licensed in their home country and possessing a VQ9 callsign issued by the local British Indian Ocean Territory representative.

[110] The navy closed the station in early 2013, and any future licensed amateurs wishing to operate from the island would therefore have had to provide their own antenna and radio equipment.

[111] The .io (Indian Ocean) country-code top-level domain was delegated by the Internet Assigned Numbers Authority (IANA) to British entrepreneur Paul Kane in 1997, and was operated for private benefit under the trade name 'Internet Computer Bureau' from 1997 until 2017.

[112] In April 2017, Paul Kane sold the Internet Computer Bureau holding company to privately held domain name registry services provider Afilias for US$70 million in cash.