The Independent on Sunday newspaper quoted a testimony by Moazzam Begg in which he talked of witnessing the killing of fellow detainees and receiving threats to his family.
Arrested in Pakistan in January 2001, Begg said his interrogation began at Bagram air base in Afghanistan and that it included being shackled and dragged, having a "suffocating hood" placed on his head and being struck on the head several times.
He also said he had witnessed two fatal beatings during interrogations by US officials in Afghanistan. "Two of these beatings resulted in the deaths of two detainees in June and December 2002. I was witness to both, in some fashion," said Begg.
He also said he was interrogated more than 250 times during his detention.
Begg's 25-page testimony was written while in solitary confinement last year at Guantanamo Bay.
He said a confession he signed at Guantanamo Bay was extracted "by coercion, and under duress".
Begg, along with Richard Belmar, Martin Mubanga and Feroz Abbasi, was returned to the United Kingdom after up to three years in detention at Guantanamo Bay.