Binyam Mohamed came to the UK as an asylum seeker in 1994
The only remaining British resident to be held in Guantanamo Bay has been charged with terrorism by a US military tribunal, the BBC has learned.
Binyam Mohamed, who denies the charge, says he was repeatedly tortured by interrogators in Morocco, where he was sent under "extraordinary rendition".
If convicted of conspiring to commit terrorism, the 29-year-old from west London could face the death penalty.
His lawyers are calling on the British government to help with his defence.
The BBC's Newsnight programme, which has learned of the charge against Mr Mohamed, also reports that the FBI opened a war-crimes file after concerns were raised about interrogation techniques being practised by other agencies at Guantanamo.
Newsnight correspondent Peter Marshall says news of the war-crimes file, which was opened six years ago, has emerged after a Department of Justice (DoJ) investigation.
US congressman Bill Delahunt says he wants the DoJ director general to appear before his foreign affairs committee next week to explain.
Newsnight also reveals that earlier this year Foreign Office officials sent an e-mail to the foreign secretary saying there were "serious allegations" that Mr Mohamed had been tortured.
They suggested that Britain should ask the US to investigate, but so far there has been no response.
Meanwhile, following claims that Britain has been complicit in the treatment of Mr Mohamed, the Foreign Office has said it cannot hand over information to defence lawyers, which might relate to intelligence operations.
Mr Mohamed says he was sent by the CIA to Morocco under what the US calls "extraordinary rendition".
He says he was brutally tortured in Morocco and Afghanistan and has also been repeatedly maltreated in Guantanamo Bay.
It is understood he has been charged with conspiring to commit terrorist offences in the US, including plotting to plant a so-called dirty bomb to spread radiation.
A similar charge against the alleged leader of the plot, Jose Padilla, was dropped when the prosecution failed to provide any evidence.
Reprieve, the legal charity which has represented British Guantanamo detainees, said it was time to end the "legal limbo" that Mr Mohamed has been in for years.
Andy Worthington, Reprieve spokesman, said they were still hoping the British government would intervene and put pressure on the US to repatriate him.
Mr Mohamed first 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.