An alleged al-Qaeda militant suspected of bombing a luxury hotel and two churches in Pakistan in 2002 was an informer for MI6, it has been claimed.
Adil Hadi al Jazairi Bin Hamlili was detained at Guantanamo Bay between 2003 and last year.
The Guardian claims to have seen secret Wikileaks files in which he is described as an al-Qaeda "assassin".
Other Wikileaks files suggest a mosque in north London served as a "haven" for Islamic extremists.
According to the files, 35 men held at Guantanamo Bay had gone to fight against Western forces in Afghanistan after being indoctrinated in Britain. The US documents identify two preachers at the Finsbury Park Mosque - Abu Hamza and Abu Qatada - as key recruiters.
Those revelations are contained in a Daily Telegraph report on separate secret files which suggested London was the hub of a global terror network.
The files, written by US military commanders, say that by the late 1990s the mosque was attracting young men from around the world, who were radicalised before being sent to training camps in Afghanistan.
It said the 35 detainees had passed through Finsbury Park Mosque as well as other centres such as Regent's Park and East London mosques and a rented room above a pub near Baker Street.
US intelligence officials said Finsbury Park served as "an attack planning and propaganda production base" for al-Qaeda.
BBC security correspondent Frank Gardner said some of the files show MI6 in a very bad light.
He said: "A lot of it doesn't surprise me... In my view they completely underestimated how dangerous recruiters and proselytisers like Abu Qatada and Abu Hamza were."
The Guardian says Mr Hamlili was described in his Guantanamo assessment file as a "facilitator, courier, kidnapper and assassin for al-Qaeda".
US interrogators believed he was also a British intelligence informer.
Sent back to Algeria
But, despite the accusations, he was never brought to trial and although he was sent back to his native Algeria last year it is not clear whether he is still in custody.
Wikileaks released the files of 759 Guantanamo detainees at the weekend.
The files are also claimed to reveal:
- That the US government suspected the BBC of being a "possible propaganda media network" for al-Qaeda, because a phone number for a corporation office was found in the possession of several suspected terrorists, according to the Telegraph. Director of BBC Global News, Peter Horrocks, has written to the newspaper, pointing out that he "strongly disagrees" with their interpretation of the files.
- That al-Qaeda had hidden a nuclear weapon in Europe for detonation should Osama Bin laden be captured
- That there were attempts by al-Qaeda to recruit workers at London's Heathrow Airport.
Mr Hamlili was captured in Pakistan in June 2003 and taken to Bagram detention centre in Afghanistan where he was interviewed by the CIA.
The CIA agents were apparently told he had been an informer for MI6 and the Canadian secret service since 2000.
But the CIA claimed he had "withheld important information from the Canadian Secret Intelligence Service and the British Secret Intelligence Service... and to be a threat to US and allied personnel in Afghanistan and Pakistan".
Khalid Sheikh Mohammed, who has apparently admitted being the mastermind of the 9/11 attacks, allegedly told his interrogators Mr Hamlili was behind a March 2002 grenade attack on a church in Islamabad, which killed five people.
Mr Mohammed also alleged Mr Hamlili was responsible for an attack on a church in Pakistan in December 2002 which killed three children.
Separate US intelligence reports said Mr Hamlili was "possibly involved" in a bombing outside Karachi's Sheraton hotel in May 2002 which killed 11 French engineers and two Pakistani citizens.