The Foreign Office (FCO) solicited the letter from the US State Department that forced British judges to block the disclosure of CIA files documenting the torture of a British resident held in Guantánamo Bay, the Observer can reveal.

The letter said that the release of papers relating to Binyam Mohamed would damage future intelligence sharing between the two countries.

A former senior State Department official said that it was the Foreign Office that initiated the "cover-up" by asking the State Department to send the letter so that it could be introduced into the court proceedings.

The revelation sparked fresh claims that the government is trying to suppress torture evidence relating to Mohamed, who is expected to be released this week after four years and flown to RAF Brize Norton, Oxfordshire.

The former senior State Department official said: "Far from being a threat, it was solicited [by the Foreign Office]." The Foreign Office asked for it in writing. They said: 'Give us something in writing so that we can put it on the record.' If you give us a letter explaining you are opposed to this, then we can provide that to the court."

The letter, sent by the State Department's top legal adviser John Bellinger to foreign secretary David Miliband's legal adviser, Daniel Bethlehem, on 21 August last year, said: "We want to affirm in the clearest terms that the public disclosure of these documents or of the information contained therein is likely to result in serious damage to US national security and could harm existing intelligence-sharing arrangements."

The letter provoked uproar when the judges in the case said it amounted to a clear threat from the US government to Britain.

David Davis, the former shadow home secretary, said the revelation that the Foreign Office solicited the letter contradicted Miliband's statement that Britain was responding to American pressure.

Davis accused Miliband of acting to "prevent his own government's embarrassment", amid allegations that the files contain evidence that UK intelligence agencies may have been complicit in Mohamed's torture.

Clive Stafford Smith, director of the legal charity Reprieve, said: "With each twist and turn, it becomes obvious that the US and the UK have to release this information."

The Foreign Office last night confirmed that it had requested the letter from the State Department, but said it was "sensible and proper" that they wanted a US statement on the case for the legal proceedings.

A Scotland Yard doctor yesterday visited Mohamed, 30, at Guantánamo's Camp Delta to ascertain whether he was fit enough to fly home.

Mohamed was seized by Pakistani authorities in 2002 and turned over to US intelligence officials as a suspected terrorist. He was shuttled between CIA-operated facilities in Afghanistan, Pakistan and Morocco before arriving at Guantánamo in 2004.