The US State Department yesterday warned that disclosure of secret information in the case of a British resident said to have been tortured before he was sent to Guantanamo Bay would cause "serious and lasting damage" to security relations between the countries.

    Stephen Mathias, a legal adviser to the department, also claimed that the "national security of the UK" would be affected by disclosure of the details of the detention and interrogation of Binyam Mohamed, 30, who is accused of conspiring with al-Qaida.

    Lawyers for the Ethiopian national have been arguing in the high court that they should have access to details of his interrogation from the time he was detained in 2002 until he was taken to Guantanamo Bay - where he is still held - in 2004. Mohamed claims that he was tortured by, among other methods, having his penis cut with a razor blade.

    In an email to the Foreign Office, which was read out to the court, Mathias said disclosure would cause "serious and lasting damage to the US-UK intelligence-sharing relationship and thus the national security of the UK".

    Ben Jaffey, for Mohamed, told the court that the US had said 44 documents would be made available to the "convening authority" in the US which will decide on Mohamed's prosecution but not to his legal representatives, Lieutenant Colonel Yvonne Bradley and Clive Stafford-Smith, of Reprieve, although both had been security-cleared in the US.

    Jaffey said there was "no movement on the central question - where was Mr Mohamed between 2002 and 2004?" Tim Eicke, for the government, said the US had made concessions by making documents available to the convening authority.

    After hearing from both sides in open court, the judges retired to hear further arguments in private. No decision was made last night but a ruling is expected tomorrow.

    Mohamed, a UK resident, was initially held in Pakistan in 2002 and was later secretly rendered to Morocco, where he claimed that he was tortured and had his penis lacerated while further threats were made. He was then flown by the US authorities to Afghanistan, where he claims he was subjected to further ill-treatment and interrogation. In September 2004, he was taken to Guantanamo Bay. He claims that all his confessions were a result of torture. He faces the death penalty.

    Last week, in the initial hearing of the case, the high court found that MI5 had participated in the unlawful interrogation of Mohamed. One MI5 officer was so concerned about incriminating himself that he initially declined to answer questions from the judges, even in private.

    Although the judges said that "no adverse conclusions" should be drawn by the plea against self-incrimination, it was disclosed that the officer, Witness B, was questioned about alleged war crimes, including torture.

    David Miliband, the foreign secretary, has provided the US with documents about the case. He has declined to release further evidence, arguing that disclosure would harm the intelligence relationship with the US.