Scotland Yard detectives are investigating allegations that members of the secret services were present when a London man was tortured while held in American custody, a High Court judge was told yesterday.
Shaker Aamer, 42, a British resident who has been in Guantanamo Bay for eight years, says that MI5 and MI6 officers travelled to Afghanistan in 2002 to interrogate him while he was first imprisoned at a US airbase.
The case is the third separate allegation of torture against the secret services being investigated by the police. Last night Mr Aamer's lawyers described the latest investigation as "potentially one of the most important criminal investigations there has been in this country".
Mr Aamer alleges that on one occasion at the Bagram Airbase prison, while an MI5 officer was present, his head was repeatedly "banged so hard against a wall that it bounced".
He also claims he was threatened with death during another interrogation when a British agent was present.
The new police investigation follows that of Binyam Mohamed who was released from Guantanamo Bay last year. Detectives are looking at claims that Britain colluded in his alleged torture during his detention in Pakistan. A third case involves MI6 and another unidentified possible victim of torture.
Mr Aamer is also a key witness in the allegations being made by Binyam Mohamed that British agents knew he was being ill-treated while held by the US.
Mr Aamer, a permanent UK resident, is married to a British national who lives with their four children in south London.
He has been detained by the US authorities since being flown to Guantanamo Bay in February 2002 but has never been charged with any offence.
His lawyers hope the Metropolitan Police criminal investigation will speed his release from Guantanamo so he can give evidence to the police in person.
Gareth Peirce, of law firm Birnberg Peirce, representing Mr Aamer, said : "Mr Aamer is a victim and key witness in that investigation – and yet where is he? He is in Guantanamo where the police can't go to interview him."
She added: "It is of central importance that everything is done to have him returned to this country."
Ms Peirce said the Government had continually asserted that it was making strenuous efforts to have him returned. But she added: "There is no diplomatic pressure being exerted. There is none. The Americans are saying: 'What pressure?'"
Yesterday his QC, Richard Hermer, described how Metropolitan Police officers went to the London offices of Birnberg Peirce on Wednesday.
Mr Hermer told the High Court: "It became apparent they are now investigating allegations raised by Mr Aamer into the alleged complicity of the UK security service in his mistreatment."
This year the High Court ordered the Foreign Secretary, David Miliband, to hand over to Mr Aamer's lawyers secret papers relating to his detention and treatment.
The case was still officially ongoing, but yesterday Mr Justice Sullivan decided that it should be brought to an end in view of the new police investigations.
The Government denies being involved in Mr Aamer's torture. Scotland Yard declined to discuss the case.
Beatings and threats: Shaker Aamer's allegation
*"After a few days of sleep deprivation, they took me to the interrogation room ... one of them, a British MI5 agent, was standing and they started talking to me in different languages and shouting ... I felt someone grab my head and start beating my head into the wall – so hard that my head was bouncing. And they were shouting that they would kill me. After this, they left the room and told me to think and tell them the truth or I would die. They left me alone with a gun on the table. I did not know what to make of this. Do they want me to kill myself, or do they want me to touch it so they can shoot me and say that I tried to shoot them? I was just sitting looking scared."