Mohamed was detained in Pakistan in 2002 and is the last remaining Guantanamo detainee with a right to return to the UK.
Last year, three British residents held at the prison in Cuba were allowed to return home and a fourth was transferred to Saudi Arabia.
Held without trial
Clive Stafford Smith, a human rights lawyer and the director of Reprieve, said: "I visited Binyam in Guantanamo just a week ago, and he is in a very bad state.
"Surely the least the British government can do is insist that no British resident be charged in a kangaroo court based on evidence tortured out of him with a razor blade.
"If Binyam's trial by military commission proceeds, all it will produce is evidence not of terrorism, but of torture, which will embarrass both the British and the American governments."
Mohamed has written to Gordon Brown, the British prime minister, to plead for help in securing his release, the British daily Independent reported on Friday.
Death penalty fears
Mohamed said he had considered suicide as a way of ending his ordeal and said he fears he could face the death penalty if convicted of terror offences.
"I have been held without trial by the US for six years, one month and 12 days," he wrote.
"Still there is no end in sight, no prospect for a fair trial."
Mohamed was born in Ethiopia in 1978 and came to Britain as an asylum seeker in 1994 aged 16.
His lawyers say he developed a drug habit while living in London and travelled to Pakistan and Afghanistan in 2001 in a bid to resolve his personal problems.