Former Guantánamo detainee Binyam Mohamed has launched an urgent legal attempt to prevent the US courts from destroying crucial evidence that he says proves he was abused while being held at the detention camp, the Guardian has learned. The evidence is said to consist of a photograph of Mohamed, a British resident, taken after he was severely beaten by guards at the US navy base in Cuba.

The image, now held by the Pentagon, had been put on his cell door, he says.

Mohamed claims he was told later that this was done because he had been beaten so badly that it was difficult for the guards to identify him.

In a sworn statement seen by the Guardian, Mohamed has appealed to the federal district court in Washington not to destroy the photograph, which neither he nor his lawyers have a copy of, and which is classified under US law.

The US government considered the case closed once Mohamed was released and returned to Britain in February. The photograph will be destroyed within 30 days of his case being dismissed by the American courts – a decision on which is due to be taken by a judge imminently, Clive Stafford Smith, Mohamed's British lawyer and director of Reprieve, the legal charity, said today .

Under US law, evidence relating to dismissed cases must be automatically destroyed. The only way to preserve the photograph is to have it accepted as a court document.

This is the aim of Mohamed's appeal and he says he needs the image as a crucial piece of evidence to fight his case against US authorities for unlawful incarceration and abuse. "That is one piece of physical evidence that I know exists of my abuse," he says in the statement, adding that it was taken in Guantánamo in 2006. After being kicked and punched, he says his guards "applied force to a pressure point on my arm, twisting the handcuffs up ... They tried to open my closed fists up by bending my fingers back one at a time." They took a picture of him when, he says, he was on the floor pinioned by the guards. He continues: "They then slammed me and my Qur'an into the fence." After he objected, he says, they "slammed me into the fence again".

He adds: "They then strapped me into a restraint chair and cut off half my beard. They then performed the humiliating 'anal cavity search', although it was painfully obvious that there was nothing to find."

Mohamed also describes how at one point he screamed and that this "made them redouble their efforts and my situation got worse".

He adds: "One [military guard] took the heel of my hand and pushed my nose up violently. One soldier pulled on my jaw. They slammed my forehead down on the concrete floor. One grabbed my testicles and punched me."

Mohamed said: "The authorities have consistently denied that I have been abused, and this is physical evidence that I am telling the truth, and they are not."

The Guardian is also writing to the court asking for the photographs to be disclosed in the interests of open justice and freedom of expression. Mohamed's lawyers and media organisations are already embroiled in a dispute in the UK high court over a refusal by David Miliband, the foreign secretary, and the US to disclose what their intelligence agencies knew about Mohamed's torture.

Mohamed was seized and held in Pakistan in 2002 before being secretly renditioned to Morocco. He was subsequently flown to Afghanistan before being sent to Guantánamo. Mohamed says he knows of other photographs taken of him in Morocco and Afghanistan, but he has not seen them. "These pictures including photos of my genitals," he said. "Although the US authorities still apparently deny it and refuse even to admit that I was rendered to Morocco, I was horribly tortured there and had a razor blade taken to my genitals". He also says he suspects that witness B – an MI5 officer who interrogated him in Pakistan in 2002 and currently the subject of a British police investigation – is being used as a scapegoat. "The main responsibility lies with those who established the policy of abuse, not with the functionaries who carried out their orders," he says in his statement.

Stafford Smith said: "It is difficult to understand the continuing policy of the Obama administration. Surely the public has the right to know the crimes committed by US personnel against a British resident like Binyam Mohamed."