Binyam Mohamed came to the UK as an asylum seeker in 1994
A British resident facing trial for terror offences at Guantanamo Bay has written to Prime Minister Gordon Brown to plead for help in freeing him.
Binyam Mohamed says he is considering suicide, the Independent reports.
He denies involvement with terrorism and writes that any evidence against him has been extracted through torture during six years' detention by the US.
The 29-year-old, from west London, is the last Guantanamo detainee with automatic right to British residency.
Downing Street has declined to comment on the letter.
Held without trial
Mr Mohamed was detained in April 2002 as he tried to return to the UK from Pakistan.
He says he was taken from there to Afghanistan and Morocco for questioning before being transferred to Guantanamo Bay in Cuba.
In his letter to No 10, he writes: "I have been held without trial by the US for six years, one month and 12 days.
"That is 2,234 days (very long days and often longer nights). Of this, about 550 days were in a torture chamber in Morocco and about 150 in the 'Dark Prison' in Kabul.
"Still there is no end in sight, no prospect of a fair trial."
The UK government formally requested Mr Mohamed's release from Guantanamo Bay in August 2007, along with that of four other British residents.
In his letter, Mr Mohamed says he was "grateful" for the government's intervention in his case, which gave him hope his ordeal might come to an end but that this had proved a "cruel hope" as he still had no prospect of release.
He adds that he feels "deeply betrayed" by the UK.
Mr Mohamed is expected to be charged with terrorism offences in the next few days and to face a US military tribunal.
Earlier this month, his legal team launched a legal bid to force the British government to release evidence for his defence.
They say the government has proof his testimony was obtained under torture and are seeking evidence he was subjected to "extraordinary rendition".
Mr Mohamed came to Britain as an asylum seeker in 1994, aged 16. His asylum claim was never finally determined but he was given leave to remain and went on to work as a cleaner in west London.
His lawyers say he travelled to Afghanistan and Pakistan in 2001 in a bid to resolve personal issues after developing a drug problem.