Chagos islanders have hailed a "big victory" after a United Nations court found that Britain illegally seized control of their Indian Ocean archipelago for the construction of a US military base 50 years ago.

The International Court of Justice said Britain's acquisition of the Chagos archipelago in the 1960s was "wrongful" and that it must "bring to an end its administration of the Chagos Archipelago as rapidly as possible."

Britain evicted about 2,000 people from the Chagos islands in the 1960s and 1970s so the United States could build a large airbase on Diego Garcia, the largest of its atolls. They and their descendants have been campaigning for the right to return home ever since.

Olivier Bancoult, chairman of the Mauritius-based Chagos Refugees Group, said the ruling made him "so happy." "It is a big victory against an injustice done by the British government for many years. We people have been suffering for many years - I am so lucky today," he said outside court.

Monday's ruling is advisory and non-binding, but carries significant symbolic weight because it came after the United Nations General Assembly asked for the court's advice on the case.

The United Kingdom paid the then-self governing colony of Mauritius £3 million for the Chagos islands in 1965. It combined them with three islands from the Seychelles to create British Indian Ocean territory, a new British Oversea Territory.