Britain and the United States are guilty of crimes against humanity in the forced displacement of indigenous people from the Chagos Islands, Human Rights Watch (HRW) said on Wednesday.
In a new 106-report, the US-based rights group added that the UK's "racial persecution, and continued blocking of their return home", with Washington's support, constituted an "ongoing colonial crime".
The two countries should provide full reparations to the Chagossian people, including their right to return to live in their homeland in the Chagos Archipelago in the Indian Ocean, HRW urged.
"The UK is today committing an appalling colonial crime, treating all Chagossians as a people without rights," said Clive Baldwin, its senior legal adviser and lead author of the report.
"The UK and the US, who together expelled the Chagossians from their homes, should provide full reparations for the harm they have caused."
London decided in 1965 to separate the archipelago from Mauritius, which was then part of the British empire, and set up a joint military base with the United States on Diego Garcia, the largest of the isles.
It continues to administer them but Mauritius, which became an independent Commonwealth country in 1968, has long fought to return the islands to its territory and has gained international support for its cause.
A 2019 International Court of Justice ruling backed its claim and said Britain should give up control of the remote archipelago.
Later that year, the UN General Assembly overwhelmingly voted in favour of a resolution recognising that "the Chagos Archipelago forms an integral part of the territory of Mauritius" and recommended Britain withdraw within six months.
London and Mauritius have begun talks over the islands' sovereignty, the Mauritian prime minister said last month, after the UK confirmed in November that it had agreed to discuss its future.
However, Foreign Secretary James Cleverly said in a written ministerial statement at the time that the countries had agreed the military base on Diego Garcia would continue to operate whatever the outcome.
HRW noted that there had been "no clear commitment to meaningful consultation with the Chagossians and to guarantee their right to reparations, including their right to return, in any settlement".
The New York-headquartered organisation interviewed dozens of people, including Chagossians and UK, US, and Mauritian officials, and reviewed numerous documents for its report.
It said it had identified three crimes against humanity: a continuing colonial crime of forced displacement; the prevention of their return home by the UK; and their persecution by the UK on the grounds of race and ethnicity.
The UK's Foreign, Commonwealth & Development Office (FCDO) responded to the claims by saying "we respect the work Human Rights Watch does around the world, but we categorically reject this characterisation of events.
"The UK has made clear its deep regret about the manner in which Chagossians were removed from BIOT in the late 1960s and early 1970s."
The US State Department did not respond to requests for comment on the HRW report.