Downing Street today rallied to the defence of David Miliband, the foreign secretary, over claims that the Foreign Office asked the US for help in suppressing crucial evidence concerning torture allegations.

Britain's Prime Minister Gordon Brown stands in front of the door of 10 Downing Street in London February 12, 2009. Regulators should be given the power to penalise banks that pay bonuses to reward short-term deal-making rather than long-term performance, Prime Minister Gordon Brown said on Thursday. (Reuters/Luke MacGregor /Britain) The prime minister's official spokesman insisted the Foreign Office had merely asked the US to "set out its position in writing" when it solicited a letter for the American authorities to back up its claim that if the evidence was disclosed, Washington could stop sharing intelligence with Britain.

The claim persuaded two high court judges earlier this month to suppress what they called "powerful evidence" relating to the ill treatment of Binyam Mohamed, the British resident being held in Guantánamo Bay.

"The Foreign Office has made clear that they asked the US to set out their position in writing for us and the court," a spokesman for Gordon Brown said.

In response to the British request, John Bellinger, the US state department's chief legal adviser, said in a letter to the Foreign Office last August: "We want to affirm the public disclosure of these documents is likely to result in serious damage to US national security and could harm existing intelligence information-sharing arrangements between our two governments."

In their judgment, Lord Justice Thomas and Mr Justice Lloyd Jones made it clear that without Miliband's claim about what they called the "gravity of the threat" from the US, they would have ordered the evidence to be revealed. Though the judges repeatedly used the word "threat", Miliband subsequently denied the US had threatened to stop sharing intelligence with Britain.

Miliband's denial last week led lawyers for Mohamed and the media, including the Guardian, to ask the judges to reopen the case on the grounds that the foreign secretary had fundamentally undermined his case. The judges agreed, against Foreign Office opposition, to reopen the case next month.

Clive Stafford Smith, director of Reprieve, the legal charity which represents Mohamed, said yesterday: "This just isn't going to go away unless both the US and the UK stop trying to suppress evidence of torture".