Terror suspects were interrogated on the British territory of Diego Garcia for weeks at a time, a senior aide in the Bush administration has said.
The island was used by the CIA to carry out "nefarious activities", Lawrence Wilkerson, Colin Powell's ex chief of staff, claimed.
Intelligence sources told him the vast military base in the British Indian Ocean Territory was a "transit site where people were temporarily housed, let us say, and interrogated from time to time".
Sir Menzies Campbell, a member of parliament's intelligence and security committee, said the latest allegations about Diego Garcia must be investigated with "full rigour".
"If true, they would constitute a very different picture of that which has previously been painted of the use to which Diego Garcia has been put by the United States."
Wilkerson told Vice News: "We didn't go there and build a prison facility and then stock it and interrogate prisoners in it, I've never heard that. What I heard was more along the lines of using it as a transit location when perhaps other places were full or other places were deemed too dangerous or insecure, or unavailable at the moment.
"So you might have a case where you simply go in and use a facility at Diego Garcia for a month or two weeks or whatever and you do your nefarious activities there."
Wilkerson said it would have been difficult for Britons on the islands to be unaware of what was happening. "I can't see how we could have used Diego Garcia for almost any function other than maybe a bounce-in and bounce-out and even that, the bounce-in and bounce-out, they'd be aware of," he said. "You can't land a helicopter there without the people on the island being aware of it."
In 2008, it was revealed that the US had secretly used the island as part of its ''extraordinary rendition'' programme. Last month, the UK's possible involvement came under fresh scrutiny after US senators published a damning report on the CIA's brutal handling of detainees in the wake of the September 11 attacks.
The present 50-year agreement allowing the US to run its vast military base ends in 2016. Before it was axed, an inquiry - led by Sir Peter Gibson - into treatment of detainees found that Britain ''may have become inappropriately'' involved in some cases of rendition - handing over suspects for interrogation - and identified 27 areas that needed further investigation.