The remaining British detainee in the Guantanamo Bay prison is on a hunger strike and says he is being treated inhumanely.
Shaker Aamer has been held at the US prison in Cuba for almost 10 years. He was cleared for release in 2007 when the Bush administration acknowledged it had no evidence against him.
The British government continues to press for his release.
Mr Aamer wants to be freed or sent for a "just and public" trial.
He was born in Saudi Arabia but has British residency, and his wife and two children live in London.
'Years of hardship'
BBC Radio 5 live's Victoria Derbyshire programme has obtained a letter, written and signed by eight detainees including Mr Aamer, alleging that they have been treated inhumanely during his detention.
In the letter, he describes himself, and fellow detainees, as "hostages."
He adds: "Inhumane treatment is taking place at the hospital among other areas, especially affecting the sick and those who are on (hunger) strike and our deprivation of real treatment, health, diet and appropriate clothing which are not provided to us, nor we are allowed to provide them for ourselves."
The letter concludes: "After these years of hardship that we have spent here - we want you to consider our cases as soon as possible and give us the right to a just and a public trial or set us free without conditions."
Responding to the letter, Col Donnie Thomas, the Joint Detention Group Commander - the senior military policeman in charge of all the camps - told the BBC: "It is totally false. I take my mission very seriously. In 19 months here, that is my mission. These detainees are treated with the highest dignity and respect and humanity."
Mr Aamer was captured in Jalalabad, Afghanistan, in December 2001. He was flown to Guantanamo in February 2002 and is one of 172 untried inmates.
A file obtained by the Wikileaks website suggests he is one of the key leaders inside the camp, organises hunger strikes, makes false allegations and was a "close associate" of Osama Bin Laden.
The Foreign Office say it continues to lobby the Obama administration over Mr Aamer's release.
Foreign Office Minister Alistair Burt told 5 live: "We have been making significant representations to the United States over a period of time.
"The prime minister has raised it, the foreign secretary has raised it, our officials have raised it, it's a matter for the United States to release Shaker Aamer who is a legal UK resident, and we continue to make what representations we can.
"It is a matter for the United States authorities to take the decision to release.
"Although it is some time since the foreign secretary raised the matter directly with Secretary of State [Hillary] Clinton, officials repeatedly have met with United States officials, we can't go into the details of the conversation for obvious reasons, these matters are confidential," he said.
"But I can assure people that the conversations do indeed take place, and we are continuing to work in accordance with our policy to see a UK resident return to the United Kingdom."
There is concern amongst British diplomats and ministers that recent US legislation means securing Mr Aamer's release any time soon is unlikely.
In a letter to Mr Aamer's constituency MP, the Conservative Jane Ellison, Mr Burt wrote: "We believe it is unlikely that Congress will be asked to consider Mr Aamer's case in the near future."
Mr Aamer has gone on a series of hunger strikes during his captivity.
The BBC's Victoria Derbyshire was told on a visit to the prison that prisoners on hunger strike will be force-fed rather than be allowed to die.
A doctor at the camp's hospital, who did not want to be named, told the BBC: "We will not allow someone to commit suicide using hunger strike. Knowing that the majority who are on it for political reasons - or they have been coerced by their brothers for political reasons.
"Usually it is to send a message to Washington," he added.
If necessary, detainees are restrained so a tube can be put up their nose and into their stomach to force feed them.