LONDON — More than three decades after British islands in the Indian Ocean were depopulated to make way for an American base, the British government today will ask the courts to ban the inhabitants from ever returning home.

The Chagos Islands, forming the British Indian Ocean Territory, are one of the world’s most isolated archipelagos, found some 2,200 miles east of Mombasa on the Kenyan coast.

At America’s request, Britain cleared the islands of all 2,000 of their inhabitants — referred to by one Foreign Office diplomat as “Tarzans and Men Fridays” — between 1966 and 1973. After this, an American naval and air base was constructed on the biggest island, Diego Garcia.

The High Court has twice given the islanders, known as the Chagossians, the right to return and Britain had initially accepted the ruling when the islanders won their first case in 2000.

But today, the government will try and overturn a second ruling in the Court of Appeal.

“The evidence points to this being done largely at Washington’s request,” said Clive Baldwin, from the Minority Rights Group, which is campaigning for the Chagossians. “After September 11, and with the island being used as a base for the wars in Afghanistan and Iraq, the military value of Diego Garcia has increased.”

When the islanders won their first case in 2000, the government accepted the ruling. Robin Cook, then the foreign secretary, said: “The government will not be appealing.” He declined to defend “what was done or said 30 years ago.”

The government issued an order allowing the Chagossians, who were deported to Mauritius, to return home, while reserving Diego Garcia for military use. But after the terrorist attacks on September 11, 2001, this policy was abruptly reversed.

Bombers operating from Diego Garcia can strike deep into the Middle East and South Asia. Naval vessels using its harbor can patrol the strategically vital waters of the Indian Ocean and the approaches to the Red Sea.