LONDON – It was one of the most shameful episodes in British post-war history: the secret expulsion of an entire population of islanders, carried out in clear violation of international law, to make way for a giant American military base.

Yesterday, after more than 30 years in exile and endless court battles, the inhabitants of the Chagos Archipelago won the right to return to their home, a group of 65 islands lost in the Indian Ocean and dominated by the American air and naval base on Diego Garcia.

In a damning verdict, the High Court in London condemned as “repugnant” the decision at American insistence to remove the 1,500 islanders in a series of expulsions between 1967 and 1973.

It overturned orders in council made by Prime Minister Blair’s administration in 2004 that reversed a previous court decision and banned anyone from living on the islands, known officially as British Indian Ocean Territory. The orders, made under the royal prerogative, allowed the government to dispense with the inconvenience of parliamentary oversight.

The judges, Lord Justice Hooper and Mr. Justice Cresswell, were scathing in their assessment of British policy, concluding: “The suggestion that a minister can, through the means of an order in council, exile a whole population from a British Overseas Territory and claim that he is doing so for the ‘peace, order, and good government’ of the territory is to us repugnant.”

The decision is a severe embarrassment to the British Foreign Office which has been put under strong pressure by the Americans to keep the Chagos islands empty save for American military personnel and guest workers on Diego Garcia. The expulsions were demanded by the Americans in a secret agreement in 1966 that saw Britain receive a discount on the Polaris submarine launched nuclear missile system in return for a 50-year lease on Diego Garcia.

American interest in the Indian Ocean grew in the 1960s as Britain’s retreat from empire threatened to produce a power vacuum in waters adjacent to the Persian Gulf. American military surveyors considered another British possession nearer to Africa, Aldabra Island, but it was ruled out because of the presence of a rare species of turtle. People, however, were not considered a problem.

The new foreign secretary, Margaret Beckett, must now decide whether to appeal against the decision or relent and allow the islanders to re-establish their homes. A Labor member of parliament and consistent supporter of the Chagossians, Jeremy Corbyn, tabled a motion in the Commons yesterday calling on the government to accept the verdict, while the Liberal Democrat member of parliament, David Heath, called for a statement on the ruling, saying: “They [the islanders] have been treated in an appalling way.”

The court heard how senior officials in the Foreign and Commonwealth Office conspired to conceal the operation, which involved the gassing of animals and the forcing of pregnant women into the hold of a merchant ship. Some miscarried after being dumped in the slums of Mauritius, where many islanders remain.

In one memorandum, the head of the diplomatic service between 1969 and 1973 and later Lord Greenhill of Harrow, Sir Denis Greenhill, described the Chagossians as a “few Tarzans or Men Fridays.”

In one file used in evidence, a diplomat wrote of his discomfort at the “whopping fibs” used to portray the islanders, who mostly earned their living as semiindentured labor on copra plantations, as temporary workers with no right of abode. The contrast in their treatment and that of the Falkland islanders 10 years later was all too apparent.

The Americans argue that allowing people back on to the islands would threaten the safety of aircraft and ships operating out of Diego Garcia, which played a central role in the attacks on Afghanistan and Iraq. But the supposed threat of missile attack or jamming has been dismissed as minimal. If the government does not exercise its right of appeal in the next 28 days, the first islanders could return in the very near future, Mr. Bancoult said.

The orders in council followed a High Court decision in November 2000 that overturned a 1971 immigration ordnance that banned the islanders from their homes. The then foreign secretary, Robin Cook, accepted the decision and set up a feasibility study into re-populating the islands. But after intense American pressure, the British government issued the orders in council. In a conciliatory gesture earlier this year, the Foreign Office chartered a ship to take 100 islanders back to their homes to tend the graves of relatives.