Lawyers for a British resident who the US government refuses to release from Guantanamo Bay have identified the existence of photographs taken by CIA agents that they say show their client suffered horrific injuries under torture.

The photographic evidence will be vital to clear Binyam Mohammed, 27, who the Americans want to bring before a Military Commission on charges of terrorism, say his lawyers.

Last week it emerged that Britain had negotiated the release of four detainees who have British residence status but Mr Mohammed, who speaks with a London accent, and at least three others are being held back.

In a letter sent to the Foreign Secretary David Miliband, Britain is urged to ask the US to stop the CIA destroying the pictures.

Clive Stafford-Smith, the legal director of Reprieve representing Mr Mohammed, said that he also knows the identity of the agents who were present when his client was allegedly beaten and tortured. Writing to Mr Miliband, he said: "Given the opportunity, we can prove that the evidence was the fruit of torture. Indeed, we can prove that a photographic record was made of this by the CIA. Through diligent investigation we know when the CIA took pictures of Mr Mohammed's brutalised genitalia, we know the identity of the CIA agents who were present including the person who took the pictures (we know both their false identities and their true names), and we know what those pictures show."

He added: "I have been privy to materials that allegedly support the finding that Mr Mohammed should be held, and while I cannot discuss some here (due to classification rules), I can state unequivocally that I have seen no evidence of any kind against Mr Mohammed that is not the bitter fruit of torture."

Reprieve says it will be pressing for criminal prosecutions against the CIA agents alleged to have carried out the torture.

Last week it emerged that the CIA destroyed hundreds of hours of videotapes showing the torture of detainees held by the US.

Binyam Mohammed was born in Ethiopia but was given leave to remain in the UK after seeking asylum in 1994. Seven years later, he travelled to Pakistan and Afghanistan where the Americans allege that he underwent training in firearms and explosives. In 2002, he was arrested by Pakistani immigration officials at Karachi airport on his way back to the UK. He says he was then taken to Morocco and tortured for 18 months, including having his penis slashed, before being sent to Guantanamo, where he still remains.

Mr Stafford-Smith added in his letter: "As you know, the only purported basis for the US holding Mr Mohammed is an allegation that he is an ('illegal') enemy combatant. Five-and-a-half years after his initial seizure, he is not currently charged in a military commission, and he has never been offered a fair trial. As you are aware, Mr Mohammed was rendered to Morocco by the CIA and tortured for 18 months in a way that was medieval.

"There can be no rational dispute that this is true. We have the CIA flight records which precisely match Mr Mohammed's version of events. He has nothing to do with Morocco, and he was not taken there by the CIA for a Club Med vacation."