Activists will hold a candlelit vigil tonight to mark 11 years since Shaker Aamer was detained without trial.
Mr Aamer is the last British resident in Guantanamo and was picked by Afghan fighters in Jalalabad on November 24 2001 before being shipped to the US prison in February 2002.
He has been held there ever since despite being cleared for release in 2007.
Mr Aamer's supporters fear for his physical and mental health, particularly as his lawyers say he been regularly tortured and locked in solitary confinement.
Reprieve director Clive Stafford Smith, who represents Mr Aamer, told the Star: "Can you imagine spending the last 11 years of your life locked up 4,000 miles from your family, without charges, cleared for release but still detained?
"And on top of that Shaker has been beaten up 260 times in the last year.
"Some people may be looking forward to a merry Christmas, but Shaker's torment is enough to make me choke on my turkey."
In a letter to Prime Minister David Cameron the Save Shaker Aamer Campaign wrote: "He has been subjected from that day on to cruel torture, injustice and abuse.
"He is still unlawfully imprisoned in Guantanamo in the harshest of conditions despite facing no charge or trial."
They point out that he has been cleared for release and that "the US secretary of defence can issue a waiver which would allow the transfer of those cleared for release to their home or a third country."
They called on Mr Cameron to urge US President Barack Obama to make good on his promise to close Guantanamo and free the prisoners already cleared for release, make a formal demand for the release and return of Mr Aamer to Britain and oppose any attempt to send him elsewhere.
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