For years the expelled people of the Chagos Islands have lobbied London to let them return.
Now the World Court has ruled the UK should give up control over the archipelago in the Indian Ocean as soon as possible.
The International Court of Justice said on Monday (February 26) that Britain wrongfully forced the population there to leave in the 1970s to make way for a U.S. air base.
The ruling is an advisory opinion and not binding.
But the United Nations court said Britain had acted unlawfully in the decolonization process.
Britain split the archipelago off from its colonial island territory of Mauritius in 1965.
Three years later it granted independence to Mauritius - minus the islands.
In the early 1970s, it evicted almost 2,000 residents to Mauritius and the Seychelles.
A military base was then constructed on the largest island, Diego Garcia, which it had leased to the United States.
Mauritius has argued that it was forced to give up the archipelago to gain independence from Britain.
But Britain maintained that Mauritius gave up the islands willingly.
Diego Garcia has played an important role in conflicts in Iraq and Afghanistan since 1991, acting as a launch pad for U.S. long-range bombers.
In 2016, Britain extended the U.S.'
Lease of Diego Garcia until 2036 and said expelled islanders couldn't go back.
The Mauritian legal team said the next step would be for the two sides to sit down and work out a handover.