Three British men held by the US in Guantanamo Bay for more than two years have compiled a report alleging abuse and humiliation while in captivity.
The document, to be released in New York on Wednesday, was being passed on to the Senate Armed Services Committee.
The accusations include beatings and one of the men, Ruhal Ahmed, claimed a US guard pointed a gun at his head, in front of a British interrogator.
The UK Ministry of Defence said it would investigate any such allegations.
"There was never any suggestion on the part of the British interrogators that this treatment was wrong"
They were then released without charge by British police.
Their experiences in captivity now form the basis of a 115-page report, Detention in Afghanistan and Guantanamo.
The allegations include:
The men allege that when a new camp commander, Maj Gen Geoffrey Miller, took charge, new practices began, including the shaving of beards, playing loud music, shackling detainees in squatting positions and locking them naked in cells.
Mr Ahmed said Foreign Office officials "did not seem to care or even ask him about the conditions".
The report says: "It was very clear to all three that MI5 was content to benefit from the effect of the isolation, sleep deprivation and other forms of acutely painful and degrading treatment, including short shackling.
"There was never any suggestion on the part of the British interrogators that this treatment was wrong."
"All the time I was kneeling with a guy standing on the backs of my legs and another holding a gun to my head "
But a spokesman said none of the Britons at Guantanamo had made it aware of any allegations of systematic abuse, either when they were detained in the camp or on their release.
A statement said: "Throughout we have sought to meet the twin objectives of pursuing the fight against international terrorism whilst safeguarding the interests of the British citizens detained aboard."
All three men said they had made either written or verbal complaints to British embassy officials while they were being held.
The trio said they had eventually wrongfully confessed to appearing in a video with al-Qaeda chief Osama Bin Laden and Mohammed Atta, one of the 11 September hijackers.
"There was not a single method that was not used to break their will "
In the report, it is understood Mr Ahmed says shortly after his capture in northern Afghanistan in 2001 he was questioned by a British interrogator, who identified himself as an SAS officer, while an American soldier held a gun to his head, threatening to shoot him.
The UK Ministry of Defence acknowledged that such behaviour is contrary to the Geneva Conventions and has promised to investigate any such allegation.
For its part, the Pentagon has dismissed the claims of abuse as a fabrication.
Lawyer Gareth Pierce told BBC News: "There was not a single method that was not used to break their will to make them confess to something they were not guilty of, and all three did."
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