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 when he was sent back to his native Algeria by President Obama.
But US interrogators believed he was also a British intelligence informer.
The revelations come as The Daily Telegraph revealed separate secret files which suggested London was the hub of global terror.
The files, also believed to come from Wikileaks, alleged the Finsbury Park mosque in north London was a "haven" for extremists.
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 under-estimated 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".
Sent back to Algeria But despite the accusations, he was never brought to trial and although he was sent back to Algeria last year it is not clear whether he is still in custody.
Wikileaks released the files of 759 Guantanamo detainees at the weekend.
They included unverified claims that al-Qaeda had hidden a nuclear weapon in Europe for detonation should Osama Bin laden be captured and 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 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 whic 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.

source;BBC News - 'Al-Qaeda assassin worked for MI6', secret cables claim