By Michael Buchanan
BBC correspondent in Washington
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.
Asef Iqbal, Ruhal Ahmed and Shafiq Rasul allege humiliating and abusive treatment
The document, which will be released in New York on Wednesday, will be passed on to the Senate Armed Services Committee.
It is believed that one of the men will claim he was abused in front of a British interrogator.
The UK Ministry of Defence says it will investigate any such allegations.
Asef Iqbal, Ruhal Ahmed and Shafiq Rasul - all from Tipton in the West Midlands - returned to Britain in March having spent more than two years in American custody - first in Afghanistan, then at Guantanamo Bay.
On their return they complained to the British media of mistreatment.
Their experiences in captivity now form the basis of a 115-page report.
They claim they were repeatedly beaten, bullied into making false statements and subjected to sexual and religious humiliations.
In the report, which the men's lawyers intend to pass on to the Senate Armed Services Committee for investigation, it is understood that Rhuhel Ahmed says that shortly after his capture in northern Afghanistan in 2001 he was questioned by a British interrogator 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.