The US must not be allowed again to use Diego Garcia, Britain's territory in the Indian Ocean, to transfer terror suspects, for combat operations, "or any other politically sensitive activity", without the explicit authority of the British government, a cross-party group of MPs insists.

Information about the extent to which the CIA used the island as a "black site" to transfer detainees is still being withheld, it suggests.

Inhabitants of Diego Garcia were forcibly removed by the Labour government in the 1960s to make way for a large US military base. The island has been used as a bomber base for air strikes against Iraq and Afghanistan. More controversially, it was secretly used as a refuelling transit stop for CIA aircraft rendering detainees to Guantánamo Bay.

In 2008, the Labour government was forced to retract assurances it had previously given to MPs about the CIA's use of the base. In its report on Thursday,, the Commons foreign affairs committee refers to a classified US Congress investigation that suggests the British government is still withholding information about the full extent of the CIA's use of Diego Garcia.

"Recent developments have once again brought into question the validity of assurances by the US about its use of Diego Garcia," say the MPs.

The committee refers to reports this year that the US Senate intelligence committee had found, after a four-year inquiry into the CIA's torture and rendition programme, that the CIA had detained "high-value suspects" on Diego Garcia and that the "black site" arrangement on the island was made with the "full cooperation" of the British government.

The Senate committee is still deciding how much of its report should be declassified.

The Commons defence committee warned that it would assess the implications for Britain and for "public confidence", in previous statements on American use of Diego Garcia.

Previous assurances that the US had not rendered terror suspects through Diego Garcia after the September 2001 attack on the US were "based upon inaccurate information", the MPs say.

In February 2008, David Miliband, then foreign secretary, said the US had in fact used Diego Garcia to render prisoners twice in 2002.

The report notes that the foreign affairs committee concluded there was "a strong moral case" for the UK to allow islanders to return to Diego Garcia.

Unless it is changed to include tighter controls on what use American forces can make of Diego Garcia, Britain's agreement with the US over the island and the rest of the Chagos archipelago, first signed in 1966, will automatically be extended in 2016 for a further 20 years.