The British security services colluded in the unlawful detention and organised the interrogation of a UK resident detained in Pakistan six years ago, the High Court in London has ruled.

Two judges have now given the Foreign Secretary David Miliband a week to reconsider his decision to withhold secret documents that could prove that Binyam Mohamed, who is being held in Guantanamo Bay, was tortured with the collusion of British security services.

Mr Mohamed, an Ethiopian refugee who moved to Britain when he was 15, is accused by a US military tribunal of conspiring with al Qaeda leaders to attack civilians. He is the 20th detainee selected for military trial at Guantanamo and has not yet entered a plea. His lawyers argue that the evidence against him was obtained by torture and should not be admissible to the military tribunals being conducted at the detention centre in Cuba.

Mr Mohamed, 30, who worked as a caretaker in London, was charged by the US with terror offences in May and could face the death penalty if found guilty.

Mr Mohamed was captured in April 2002 in Pakistan. He claims he spent 18 months in Morocco and was tortured there before being flown to an alleged CIA-run site in Afghanistan and later transferred to Guantanamo, where he has spent the past four years.

The High Court heard the case over five days in August and only some sessions were open to the public because of the sensitivity of the information. During the hearing Dinah Rose QC, for Mr Mohamed, said he was tortured after his detention in Pakistan. He was then rendered to Morocco where he was subjected to prolonged and brutal torture after being made to "disappear", she said.

The former Kensington caretaker alleges he was repeatedly slashed in the genitals with a razor blade while being held in Morocco.

Ms Rose told judges the US authorities denied that Mr Mohamed had been subjected to extraordinary rendition or torture. But there were strong grounds for believing that MI6 and MI5 held independent evidence supporting his story.

In their ruling the judges stated that the Foreign Secretary accepted that Mr Mohamed had established an arguable case that he was "subject to torture during his detention by or on behalf of the United States". The court had concluded that the conduct of the Security Service "facilitated interviews by or on behalf of the United States when Binyam Mohamed was being detained by the United States incommunicado and without access to a lawyer in Pakistan in the period April 2002 until at least May 17, 2002 when he was seen by an officer of the Security Service".

Lawyers for the Foreign Office argued at the hearing that the government had acted within its powers and was not legally obliged to make the disclosures sought. The Foreign Secretary was entitled to proceed on the basis that the US legal system would safeguard Mr Mohamed's rights.

Clive Stafford Smith, director of the human rights charity Reprieve who has represented Mr Mohamed since 2005, described the ruling as a "momentous decision". He said: "Compelling the British government to release information that can prove Mr Mohamed's innocence is one obvious step towards making up for the years of torture that he has suffered."

The ruling did acknowledge that the British government last year requested that Mr Mohamed be returned to the UK and had gone to "great lengths" to assist him.

A spokesman for the Foreign Office said: "For strong reasons of national security, to which the court accepted we were entitled to give the highest weight, we could not agree to disclose this information volunarily."