The Foreign Office stands accused of misleading the public over the plight of thousands of islanders who were expelled from their Indian Ocean homeland to make way for a large US military base.

More than 2,000 islanders – described privately by the Foreign Office as "Man Fridays" – were evicted from the British colony of Diego Garcia in the 1960s and 70s. The Foreign Office, backed by the US, has fought a long legal battle to prevent them returning home.

The islanders' quest to go back will be decided by a ruling, expected shortly, from the European court of human rights.

A secret US diplomatic cable released by WikiLeaks suggests the Foreign Office has privately admitted its latest plan to declare the islands the world's largest marine protection zone will end any chance of them being repatriated.

The admission is at odds with the public position of the Foreign Office that the proposed park does not prejudice or have any effect on the the islanders' right to return. The islanders have claimed the marine park was a ploy to block their return, saying it would make it impossible for them to live there as it would ban fishing, their main livelihood.

The disclosure follows years of criticism levelled at Whitehall over the harsh treatment of the islanders, many of whom have lived in poverty in other countries since their deportation.

In the past, National Archive documents have revealed how the Foreign Office consistently lied about the eviction, maintaining the fiction that the islanders had not been permanent residents.

The latest leaked documents are US state department cables recording private meetings between Foreign Office mandarins and their American counterparts.

In May 2009, Colin Roberts, the Foreign Office director of overseas territories, told the Americans Diego Garcia's value in "assuring the security of the US and UK" had been "much more than anyone foresaw" in the 1960s, when the plan to set up the base was hatched.

"We do not regret the removal of the population since removal was necessary for [Diego Garcia] to fulfil its strategic purpose," he added under a passage that the Americans headed "Je ne regrette rien".

Roberts, admitting the government was "under pressure" from the islanders, told the US of the plan to set up the marine park on 55 islands around Diego Garcia, known as the British Indian Ocean Territory (BIOT). "Roberts stated that, according to [Her Majesty's government's] current thinking on a reserve, there would be 'no human footprints' or 'Man Fridays' on the BIOT uninhabited islands," according to the American account of the meeting. The language echoes that used in 1966 when Denis Greenhill – later the Foreign Office's most senior official – described the inhabitants as "a few Tarzans and Man Fridays".

The leaked cable says Roberts "asserted that establishing a marine park would, in effect, put paid to resettlement claims of the archipelago's former residents".

A US state department official commented: "Establishing a marine reserve might, indeed, as the FCO's Roberts stated, be the most effective long-term way to prevent any of the Chagos Islands' former inhabitants or their descendants from resettling in the BIOT."

The Foreign Office said it rejected any allegation that it had misled the public. A spokesman added : "We cannot go into specifics of any leaked documents because we condemn any unauthorised release of classified information."

Leading conservation groups have supported the marine park plan. Roberts is quoted as telling the Americans that Britain's "environmental lobby is far more powerful than the [islanders'] advocates".