The High Court has released details of a document suggesting attempts were made by the US government to prevent Guantanamo detainee Binyam Mohamed taking legal action over torture allegations.
The document relates to plea bargaining discussions last year.
UK resident Mr Mohamed was eventually released from Guantanamo Bay earlier this year and allowed to return home.
But his lawyers are still fighting to prove that he suffered ill-treatment.
The Ethiopian national alleges that he was tortured and interrogated during more than six years in detention as a terror suspect. He says his ordeal included rendition to Morocco where he was held and cut with a scalpel on his chest and penis.
Reprieve director Clive Stafford Smith, who has represented Mr Mohamed for four years, said after the release: "The facts revealed today reflect the way the US government has consistently tried to cover up the truth of Binyam Mohamed's torture.
"He was being told he would never leave Guantanamo Bay unless he promised never to discuss his torture, and never sue either the Americans or the British to force disclosure of his mistreatment. Gradually the truth is leaking out, and the governments on both sides of the Atlantic should pause to consider whether they should continue to fight to keep this torture evidence secret."
Last year Mr Mohamed's lawyers became engaged in a series of High Court battles to obtain access to 42 documents they argued supported his case against the US authorities. In October, Lord Justice Thomas and Mr Justice Lloyd Jones ruled at London's High Court there was a "clear evidential basis" for accusations that the US government was doing "all it could" to avoid disclosing the documents.
They referred to "delays and unexplained changes of course" by the American authorities, and said the documents were "essential" for Mr Mohamed's defence. But they concluded it was for the US courts to resolve the issues raised.
They said it was now "appropriate" to reveal details of the agreement in an annex to the judgment consistent with "principles of open justice and the rule of law".
