The International Court of Justice ruled in an advisory opinion that is non-binding that Britain had unlawfully carved up Mauritius, which the Chagos were a part of, when it agreed to end colonial rule in the late 1960s.

The court's opinion, however, carries weight since it came from the top UN court and could put significant pressure on London to act.

The islands are currently known as the British Indian Ocean Territory. Britain has in the past paid compensation to islanders, but the court says that London now needs to go further.

The UK government reacted to the ruling on Monday by saying it will look "carefully" at the ruling while stressing that the court's view is "an advisory opinion, not a judgment".

Britain evicted about 2000 people from the Chagos archipelago in the 1960s and 70s so the US military could build an air base on Diego Garcia. Many resettled in the UK and have fought in British courts to return to the islands.

One of them was Olivier Bancoult, who said he and his family were forcibly moved to Mauritius when he was just four years old.

"Oh, I feel so happy today because it's a big victory against injustice done by the British government for many years," he said outside court.

"I am so lucky today and to have this opinion delivered by the International Court of Justice - to say what has been done to us and what has been done to Mauritius is unlawful," he added.

The court said it concluded that even though Mauritius' government agreed to split off the Chagos islands shortly before the country's 1968 independence, "this detachment was not based on the free and genuine expression of the will of the people concerned".

The court said that it is up to the UN General Assembly to decide how to "ensure the completion of the decolonisation of Mauritius".

The opinion was delivered by a majority of 13 of the court's 14 judges, with the US judge, Joan E. Donoghue, dissenting.

The US still has a military base on Diego Garcia, but the court said that in its opinion "all member states are under an obligation to co-operate with the United Nations in order to complete the decolonisation of Mauritius".

Britain's Foreign Office said in a statement that "the defence facilities on the British Indian Ocean Territory help to protect people here in Britain and around the world from terrorist threats, organised crime and piracy".

At hearings last year, Mauritius Defence Minister Anerood Jugnauth said the US base would not be threatened by the outcome, telling judges that "Mauritius recognises its existence and has repeatedly made it clear to the United States and the administering power that it accepts the future of the base".

Philippe Sands, a lawyer who represented Mauritius, hailed the decision which he said would put pressure on Britain to act.

"One hopes that the two countries will sit down and work out a sensible way of taking this forward," Sands said.

"Relations between Mauritius and the UK are excellent; relations between Mauritius and the US are excellent. They will continue to be excellent. A way will be found to move this forward, safeguarding the interests of everyone but recognising what the court has said."

Australian Associated Press