The security service, MI5, is to be investigated by the Metropolitan Police over allegations that its officers were complicit in the torture of a UK resident held by the US government for seven years.
It is believed to be the first time that the police have publicly investigated the secret services.
The Attorney General, Lady Scotland, asked Scotland Yard to investigate torture allegations made by the former Guantanamo inmate Binyam Mohamed, 30, during his detention in Pakistan, Morocco and Afghanistan between 2002 and 2004.
Any evidence to support torture or war crime charges will be passed to the Director of Public Prosecutions, who will decide whether a case can be brought to trial.
Last night, Mr Mohamed's lawyers, human rights groups and politicians called for the investigation to be as wide-ranging as possible so that all those who knew about Britain's alleged collusion in torture, including ministers and foreign agents, could be held to account.
Mr Mohamed, 30, an Ethiopian national granted British residency, was released from the US naval base last month after seven years in detention.
He was arrested at Karachi airport in Pakistan in 2002 while trying to return to Britain following a trip to Afghanistan. He alleges that he was tortured at a series of secret prisons in Pakistan, Morocco and Afghanistan, being beaten, scalded and having his genitals slashed with a scalpel.
In a statement made after he returned to Britain, he said "the very worst moment came when I realised in Morocco that the people who were torturing me were receiving questions and materials from British intelligence".
Yesterday, Mr Mohamed said he was "very pleased" that an investigation would now take place. But he made it clear that the MI5 agent who interviewed him in Pakistan should not carry the can for a wider policy: "I feel very strongly that we shouldn't scapegoat the little people. We certainly shouldn't blame 'Witness B' [the MI5 agent also known as John] – he was only following orders."
Mr Mohamed's claims were originally referred to Lady Scotland last year by the Home Secretary Jacqui Smith after they surfaced in a High Court case brought by his lawyers. Lady Scotland said that she and the Director of Public Prosecutions, Keir Starmer QC, had reviewed a "substantial body of material" relating to the case, including the testimony of the MI5 officer identified as Witness B.
According to his note of an interview he conducted with Mr Mohamed in Pakistan in 2002, Witness B offered to use his influence with the US authorities to help him if he was co-operative.
"I told Mohamed he had an opportunity to help us and help himself. The US authorities will be deciding what to do with him and this will depend to a very large degree on his degree of co-operation," Witness B noted, according to evidence to the High Court.
"I said that if he could persuade me he was telling the complete truth I would seek to use my influence to help him. He asked how and said he did not expect ever to get out of the situation he was in.
"I said it must be obvious to him he would get more lenient treatment if he co-operated. I said that I could not and would not negotiate upfront but if he persuaded me he was co-operating fully, then (and only then) I would explore what could be done for him with my US colleagues."
According to court documents, Witness B made clear under cross-examination that he had been acting with the full authority of his superiors in MI5. "I was always, whenever conducting an interview, careful to make sure that I had the clearance of my management to proceed and I did in this case," he said. "I was aware that the general question of interviewing detainees had been discussed at length by Security Service management legal advisers and Government and I acted in this case, as in others, under the strong impression that it was considered to be proper and lawful."
Zachary Katznelson, the legal director of the charity Reprieve, which represents Mr Mohamed, said he was concerned that secret evidence would be excluded from the investigation.
"The Attorney General absolutely did the right thing today," he said. "It is critical we get to the bottom of what was done to Binyam Mohamed and the role of any British official in his torture.
"But for this to be a proper inquiry the police have to be given access to all the information and that includes any secret information. Many of the documents related to Mr Mohamed's treatment have been classified either in the US or the UK and unless the police have access to all of them they will only see one tiny piece of the picture."
Reprieve also wants the investigation to consider the evidence against CIA and Moroccan agents who are alleged to have tortured Mr Mohamed – and to seek extradition if appropriate.
The MI5 agent: His chain of command
* Witness 'B', MI5 agent. Interviewed Binyam Mohamed in Pakistan in 2002.
* Witness B's controllers. Telegrams between MI5 and the CIA show Witness B's superiors met at MI5 headquarters to discuss Mohamed's case, suggesting they knew he was held by a third party while they supplied questions. Witness B said that his superiors had cleared his interview.
* Head of MI5. Mohamed was accused of planning a bomb attack on the US. This would likely have been brought to the head of MI5. Sir Stephen Lander was director general from 1996 to 2002, then Elizabeth Manningham-Buller until 2007. She said in 2006 that British services do not ask if intelligence was obtained by torture as "that would make things difficult".
* David Blunkett, Home Secretary in 2002. The head of MI5 briefs ministers on the terror threat. Because of the seriousness of the case it is possible the Home Secretary was aware.
* Jack Straw, Foreign Secretary in 2002. The Foreign Secretary often clears joint US intelligence and interrogation operations.