An al-Qa'ida operative accused of bombing Christian churches and a luxury hotel in Pakistan in 2002 was working for British intelligence at the same time, according to leaked files.
The claim about Adil Hadi al Jazairi Bin Hamlili is made in secret reports on detainees at the US military's Guantanamo Bay prison camp.
The Algerian citizen, captured in Pakistan in 2003, was described by interrogators as a "facilitator, courier, kidnapper, and assassin for al-Qa'ida".
But the Americans also believed he had "withheld important information from the Canadian Secret Intelligence Service and British Secret Intelligence Service ... and (was) a threat to US and allied personnel in Afghanistan and Pakistan".
The files, handed to the Guardian and Daily Telegraph by whistleblowing website WikiLeaks, also indicate that at least 35 terrorists incarcerated at Guantanamo Bay went to fight against the West after being radicalised by extremist preachers in the UK.
Abu Qatada and Abu Hamza are identified in the documents as key recruiters responsible for sending dozens of extremists from throughout the world to Pakistan and Afghanistan via London mosques.
It also appears that the US government suspected the BBC of being a "possible propaganda media network" for al-Qa'ida after a phone number for the World Service was found in the possession of several suspected terrorists.
But a spokeswoman for the corporation said: "Independence and impartiality are at the heart of all BBC World Service output.
"The service has interviewed representatives of organisations from all sides involved in the Afghan conflict so it would not be surprising that a number believed to relate to the BBC Pashto service was in circulation."