The high court yesterday condemned as "deeply disturbing" a refusal by the US to disclose evidence that could prove a British resident held at Guantánamo Bay was tortured before confessing to terrorism offences.

The court said there was "no rational basis" for the American failure to reveal the contents of documents essential to the defence of Binyam Mohamed, who faces the death penalty.

In a particularly damning passage, Lord Justice Thomas and Mr Justice Lloyd Jones said claims by Mohamed's lawyers that the US was refusing to release the papers because "torturers do not readily hand over evidence of their conduct" could not be dismissed and required an answer.

The judges said they were unaware of any precedent for such serious allegations against "the government of a foreign friendly state and our oldest and closest ally" as those made in this case.

The US had not provided any explanation for its conduct, though it had had "ample time" to do so, the judges said.

Thomas and Lloyd said the documents provided the "only independent evidence" capable of helping Mohamed and his defence. Suppressing the material "would be to deny him the opportunity of timely justice in respect of the charges against him", which was a principle dating back to "at least the time of Magna Carta and which is a basic part of our common law and of democratic values".

They said David Miliband, the foreign secretary, conceded there was an "arguable case" that Mohamed had been subjected to torture and inhuman treatment. Yet Miliband also wanted to suppress relevant documents, not because they would reveal any intelligence operations but because the US claimed that if they were disclosed serious harm would be done to "intelligence sharing" between the UK and the US.

The judges said it was clear Britain had "facilitated" Mohamed's interrogation when he was unlawfully detained in Pakistan before being secretly rendered to Morocco, Afghanistan, and then to Guantánamo. The US was using confessions made after two years of unlawful "incommunicado detention" on charges where the death penalty might be sought, the judges said yesterday.

They noted that a military prosecutor at the US base had recently resigned in protest against the treatment of prisoners, including the use of a "frequent flyer programme". The judges described this as a "euphemism for a sleep deprivation programme". They added: "This is a practice which the United Kingdom expressly prohibits."

Charges against Mohamed- including that he was involved in a dirty bomb plot - have been dropped, allegedly to prevent the US from revealing torture evidence. The US authorities now planned to charge him with other offences, the judges noted yesterday.

The judges took the extraordinary step of inviting the media to challenge previous decisions to hold many of the case's hearings in camera. "Although the argument took place in closed session," they said, "the issue is one of considerable importance in the context of open justice [and] to the rule of law."

They suspended proceedings pending a case in the US courts, where defence lawyers are also trying to force disclosure. That federal court, the British judges said yesterday, might be given explanations about US conduct "denied to this court".

Clive Stafford Smith, the director of the charity Reprieve, described Mohamed's treatment by the US as a "litany of misconduct".

"First they tortured him, then they held him for more than six years without trial, now they want to cover up evidence that could set him free," he said. "What is the point of a 'special relationship' if the UK government cannot secure basic justice for Mr Mohamed?"

Richard Stein, one of Mohamed's lawyers, said: "The grave concerns expressed by the court about the dealings of the Americans in this case are not surprising, given the torture Mr Mohamed has suffered. This underlines the British government's duty to do more than gently nudge its ally across the Atlantic when it comes to criminal acts taken against a British resident."

Mohamed, 30, an Ethiopian national and British resident, was held in Pakistan in 2002, when he was questioned by an MI5 officer. He was later secretly rendered to Morocco, where he says he was tortured by having his penis cut with a razor blade. The US subsequently flew him to Afghanistan and he was transferred to Guantánamo Bay in September 2004.

Backstory

Attempts to get the UK courts and parliament to take notice of the case of Binyam Mohamed began more than three years ago. Clive Stafford Smith, director of the charity Reprieve, told the Guardian about a hunger strike by Guantánamo Bay prisoners, including Mohamed, who was rendered to the US base in Cuba. Mohamed's case was taken up by the all-party parliamentary group on extraordinary rendition, chaired by Andrew Tyrie MP

UK citizens and residents held at Guantánamo were released between 2005 and 2007, but Mohamed was kept. This summer, the high court heard about the way the US and British governments tried to stop the release of evidence about his case. Defence lawyers argued that MI5 misled MPs about his treatment. In August, in an interim judgment, the high court ruled that MI5 had participated in Mohamed's unlawful interrogation.