Ministers are to face a legal challenge over the use of secret guidance given to members of the secret services when interrogating prisoners abroad.
Lawyers and human-rights groups claim the unpublished codes of practice, issued in 2002 and 2004, allowed officers working for MI5 and MI6 to collude in torture. Reprieve, which represents Binyam Mohamed and several other former Guantanamo Bay detainees, is to go to court to force the Government to disclose what it describes as Britain's "torture policy".
The Metropolitan Police is investigating three allegations of torture concerning MI5 and MI6. Last night, the Attorney General, Baroness Scotland, declined to ask the police to investigate five more cases involving British citizens imprisoned in Afghanistan.
The claims were made in a November 2009 report by Human Rights Watch, and were referred to the Attorney General by the Tory MP David Davis. Ministers have said that the UK neither practises nor condones torture or the mistreatment of prisoners.
But Reprieve has accused the Government of making "broad assertions" that its policy is legal while failing to address specific allegations. Evidence from 10 cases suggests the policy could have unlawfully endorsed complicity in torture, it claims. Gordon Brown has yet to fulfil a pledge made last year to release the latest version of the guidance.
Reprieve director Clive Stafford Smith accused the Government of "playing for time". He said: "Advice given to agents cannot sensibly be deemed 'classified' as disclosing legal advice hardly betrays a national secret. Rather, depending on what the policy was, it exposes those who sanctioned the advice to immense embarrassment.
"Equally, it cannot take a year to come up with new advice – we could have written it for them in an afternoon. Agents in the field are still, apparently, required to rely on the 2004 policy."
A Cabinet Office spokesman said the Prime Minister's "unprecedented commitment" to publish the new 2009 guidance was "the exception to the rule". He said: "Recognising the importance of this particular issue, the Prime Minister has made an unprecedented commitment to publish consolidated guidance in order to make plain the standards to which we hold ourselves."
Scotland Yard is currently investigating a claim that MI5 was complicit in the torture of the former Guantanamo Bay detainee Binyam Mohamed, who was held in Pakistan, Morocco and Afghanistan.
Detectives are also looking into claims that MI6 was complicit in the torture of a non-Briton in a separate case. It was reported last week that the police are also investigating a third case involving Shaker Aamer, the last British resident still held at Guantanamo Bay.