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Between 1968 and 1973 the British Government cleared the entire Chagos Archipelago of its inhabitants in anticipation of a US military base on the biggest island, Diego Garcia. The Archipelago was made a colony, the British Indian Ocean Territory (BIOT). It subsequently became a British Overseas Territory.

Two main disputes have arisen from these events. One has been between the Chagos Islanders and the British Government over the legality of their removal and whether they have a right to return. The other has been between the UK and Mauritius about sovereignty over the BIOT. In 1965 the UK undertook that it will cede sovereignty to Mauritius once the BIOT is no longer required for defence purposes.

A May 2013 Library briefing surveyed the origins and subsequent evolution of these disputes, including past and ongoing legal cases brought before British, European and international courts. This paper summarises the main developments since May 2013. It is an update to a briefing previously published in December 2019.

Chagossian resettlement?

In June 2015 the Supreme Court heard an application for the 2008 verdict of the House of Lords – which ruled that the use in 2004 of Orders in Council to prevent the Chagossians from returning had been lawful – to be set aside. In June 2016 the Supreme Court ruled against the application by a majority of three to two. But, with a fresh resettlement feasibility study recently completed, it also said that the Government should reconsider the ban on the Chagossians returning home as it may no longer be lawful.

However, in November 2016 the previous Government announced that it had decided not to allow resettlement. The decision provoked condemnation from Mauritius and supporters of the Chagossian cause in the UK and became the subject of judicial review. In February 2019, the Divisional Court found for the Government. Lawyers for the Chagossians sought permission to appeal the verdict, which was granted in July 2019.

That appeal was heard in May 2020 and a judgement was passed down in July. The appeal centred around the applicability of the European Convention on Human Rights (ECHR) to the Chagos Islands, whether the Divisional Court had applied the right level of scrutiny to the Government’s decision making, and the rationality of the decision made by the Government in 2016. The appeal was dismissed on all grounds.

There has also been ongoing debate about the best use of – and motives behind – a £40 million support package for Chagossians, to be spent over a ten-year period, which was announced when the decision on resettlement was made.

Status of the Marine Protected Area

In 2015 a Marine Protected Area (MPA), previously introduced by the British Government around the Chagos Archipelago (apart from Diego Garcia, where there is a US military base), was ruled by a Tribunal under the UN Convention on the Law of the Sea to have been established without proper regard to the rights of Mauritius. Some legal commentators have taken this as tantamount to saying that the MPA is unlawful. The UK disagrees with this interpretation.

The UN Tribunal was more equivocal on sovereignty than the Mauritian Government, which brought the case, hoped it would be. It declined jurisdiction on the issue but found that the UK’s 1965 undertaking to cede sovereignty to Mauritius when the BIOT is no longer required for defence purposes is binding under international law.

Chagossians in the UK have been seeking to challenge the establishment of the MPA through the English courts since 2013. They argued that it was created for an improper purpose – to prevent resettlement – and that the consultation process prior to its establishment had been inadequate. However, the High Court and Court of Appeal found against the Chagossians and in February 2018 the Supreme Court did so too.

Sovereignty dispute

In July 2016 Mauritius said that it would seek a referral by the UN General Assembly to the International Court of Justice, in order to obtain an Advisory Opinion on sovereignty. Advisory Opinions are not legally binding but can carry great legal weight.

Mauritius put the matter before the General Assembly in June 2017, which referred the matter to the Court. The ICJ issued an Advisory Opinion in February 2019, in which it found that the “process of decolonization of Mauritius was not lawfully completed when that country acceded to independence in 1968, following the separation of the Chagos Archipelago”. It went on to assert that the UK is therefore “under an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible.”

The UK Government rejected the Advisory Opinion but said it was open to further bilateral talks with Mauritius. On 22 May 2019, the UN General Assembly passed a resolution which endorsed the Opinion and called on the UK to end its administration of the BIOT within six months. The UK Government did not accept this resolution. The six-month deadline passed on 22 November without UK acknowledgement. Mauritian prime minister Pravind Jugnauth subsequently called the UK an “illegal colonial occupier”. The Mauritian Government indicated that it was exploring the possibility of bringing crimes against humanity charges against individual British officials at the International Criminal Court (ICC). In October 2020 a submission was filed with the ICC accusing the BIOT Commissioner, Deputy Commissioner, and the UK’s military representative BIOT, of apartheid.

In January 2021 a Special Chamber of the International Tribunal for the Law of the Sea ruled in a dispute between Mauritius and the Maldives regarding the delimitation of their maritime boundary. The Tribunal rejected arguments that it had no jurisdiction in the case and subsequently determined that Mauritian sovereignty over the Chagos Archipelago had been inferred by the 2019 ICJ Advisory Opinion. The UK has rejected the ruling, arguing that as a non-party to the ITLOS dispute, its ruling can have no legal effect on the UK. However, there is a broad consensus among commentators that the ruling puts greater international legal pressure on the UK Government. It has also led many to, once again, question the legality of the Marine Protected Area.

US military base on Diego Garcia

In November 2016 the decision was made to extend the US military base on Diego Garcia until 2036.

The Biden administration has not made any public statements about the US’ military presence in Diego Garcia, or the wider sovereignty dispute between the UK and Mauritius. Given the number of challenges facing the new administration, both domestically and abroad, it is uncertain whether the BIOT sovereignty issue will be high on Biden’s agenda.

However, efforts to highlight the sovereignty dispute have reportedly been made to the new administration by the Mauritian Government and Chagossian supporters. The Mauritian Government has offered to lease the base to the US for 99 years, far longer than the current deal for access to 2036.


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