The last British resident held in Guantanamo Bay claims he is subjected to physical abuse and fears he will not make it out alive.
Shaker Aamer has warned his wife and children in London that he may die in captivity and is currently on hunger strike protesting against abuse by his guards, the Mail on Sunday reports.
The 46-year-old is to set to be freed after more than 13 years in detention without charge.
He cannot be released immediately because the US administration had to give Congress 30 days' notice of his release.
"I know there are people who do not want me ever to see the sun again," he told the Mail on Sunday.
"It means nothing that they have signed papers, as anything can happen before I get out.
"So if I die, it will be the full responsibility of the Americans."
Mr Aamer has made a series of allegations about torture and abuse at the military prison in Cuba.
He is said to be on hunger strike after he was allegedly assaulted by guards and forced to give blood samples.
Mr Aamer described being held in a freezing cold aircraft hangar during plummeting temperatures in winter in Afghanistan.
He also claims a British intelligence officer was present when his head was repeatedly beaten into a wall by the Americans during interrogations.
Two years ago Mr Aamer submitted a witness statement to the Metropolitan Police detailing his ordeal, according to the Mail on Sunday.
Mr Aamer reportedly told detectives he was "abused by the US military from the day I arrived".
The British resident, who has a wife and four children living in Battersea, south London, has said he was originally seized by bounty hunters while working as a charity worker in Afghanistan in 2001 shortly after the 9/11 attacks.
He was handed over to US forces and in February 2002 he was transferred to Guantanamo Bay and accused of aiding al Qaeda.
Mr Aamer has been accused of being associated with Osama bin Laden but was never charged.
In 2007 the allegations were dropped and he was cleared for release but, despite a formal request for his return by then foreign secretary David Miliband, the US authorities refused to allow him to go.