"If there is any further and wider criminal investigation ... I believe it would be completely impossible to decide that there has not been a pattern of massive complicity by UK bodies in criminality at the highest levels directed at other Muslim prisoners," Mohamed said. "My experience was not isolated; it was part of a pattern."
Eliza Manningham-Buller, a former head of MI5, has said she believes the U.S. deliberately misled its allies over its handling of detainees during the so-called war on terror.
In a separate allegation of complicity from a former detainee, investigators also say they failed to find sufficient evidence — mostly because they lacked access to witnesses and the detainee who had been held by U.S. authorities at the Bagram Air Base in Afghanistan.
Some 3,000 terror suspects continue to be held at the secretive detention facility where detainees lack access to lawyers. Human rights organizations have repeatedly criticized U.S. authorities for a lack of transparency and legal protection for the detainees.
"One thing you read very clearly in those materials is that it is not that there wasn't torture, it is not that the British weren't involved, it is that there are witnesses who are not available to put their part," said Clive Stafford Smith, a lawyer with the legal charity Reprieve who represents some of the alleged victims of torture and rendition.
Most of the torture allegations came from terror suspects who were either initially held in Pakistan and Afghanistan, or sent to other countries such as Morocco for interrogation.
British agents were accused of passing on information about detainees but not of direct abuse.
Britain has already made payouts to 16 former detainees at Guantanamo. Among those alleged to have been part of the settlements were Mohamed, Bishar Al Rawi, Jamil El Banna, Richard Belmar, Omar Deghayes, Moazzam Begg and Martin Mubanga
British prosecutors and police said that while there was insufficient evidence to bring criminal charges now, cases could be reopened if new evidence emerges.
A separate government inquiry into Britain's role in the so-called war on terror is expected to begin later this year.