Campaigners will use the 11th anniversary of Shaker Aamer's incarceration without trial in Guantanamo Bay this week to renew their demands for his release.
Mr Aamer was arrested by Afghan forces on November 24 2001 in Jalalabad and subsequently transferred to US custody.
Originally from Saudi Arabia, Mr Aamer had permission to live indefinitely in Britain on the basis of his marriage to a British national at the time of his original detention.
Mr Aamer has alleged he was subjected to torture including severe beatings and other ill-treatment at the hands of US officials.
He also claims MI5 officers were present at interrogations during which his head was "repeatedly banged so hard against a wall that it bounced."
Since his transfer to Guantanamo, Mr Aamer has repeatedly alleged that he has also been tortured there.
Mr Aamer, who speaks fluent English, has been involved in protests against conditions at the camp, including participation in hunger strikes and speaking out on behalf of his fellow detainees.
As a result, his lawyers say, he has been subjected to prolonged isolation and frequent ill-treatment.
Amnesty International UK director Kate Allen said: "The continued detention of Shaker Aamer is totally baffling and the issue has become a running sore in UK-US relations.
"He faces no charges, was officially approved for transfer out of the camp years ago and the UK has said it will accept him back - so what is the hold-up?
"Given the time involved, the lengthy spells in solitary confinement and the torture allegedly used against him, Shaker's plight illustrates why Guantanamo has become such an affront to human rights."
London-based Save Shaker Aamer Campaign has once more written to Prime Minister David Cameron and other senior coalition figures urging further efforts on their part.
It said: "The UK government has requested Shaker Aamer's release and return to the UK in informal talks with the US. To date, these talks have not been successful. Informal talks are not enough.
"The government must find ways to require the US to release and return this British resident to his family. We demand that the government stands up for Shaker. Don't stand by and allow Shaker to suffer this gross injustice."
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