The Mauritius president has threatened to leave the Commonwealth in protest at the UK's "barbarous" treatment of the people of the Chagos Islands.
The islanders were exiled to make way for a US airbase
Sir Anerood Jugnauth says he may take the UK to the International Court of Justice over the islanders' plight.
The Chagos Islands, a British colony in the Indian Ocean, were leased to the US in the 1960s to build a military base.
The residents were forced out, and the government says they cannot return, but have been granted UK citizenship.
Many of the residents now live in poverty in Mauritius, or as refugees in the UK.
Many have fought for the right to return to the islands, or for more help to improve their living standards in the UK.
The American base was built on the large island of Diego Garcia within the Chagos archipelago.
Mauritius claims the islands as part of its territory, and Mr Jugnauth suggested his country was forced by the British to accept the Chagossians as a condition of independence.
The Diego Garcia base was crucial during the Cold War and has gained new significance in recent years as a key launching point for bombing missions in Iraq and Afghanistan.
In 2000, the courts ruled that Chagossians could return to their homes in 65 of the islands, but not to Diego Garcia. Then Foreign Secretary Robin Cook said the government would not appeal.
But in 2004 the government used the royal prerogative, powers that allow action without reference to Parliament, to effectively nullify the decision.
Sir Anerood Jugnauth is appalled by the UK's stance
But last year the High Court overturned the order and rejected government argument that the royal prerogative, exercised by ministers in the Queen's name, was immune from scrutiny.
The government's behaviour towards the Chagossians was described as "repugnant".
Sir Sydney Kentridge QC said then that there was no known precedent "for the lawful use of prerogative powers to remove or exclude an entire population of British subjects from their homes and place of birth".
Now the government has taken the case to the Court of Appeal, saying it prevents legislation relating to security and seriously affects the legal relationship with overseas territories.
A Foreign Office spokeswoman said: "We regret the decisions that were made in the 1960s and 1970s but it is important to remember that the vast majority of Chagossians are either citizens of Mauritius or the Seychelles. They are also British citizens with the right of abode in the UK."