The double agent who helped foil an al Qaeda plot to blow up a US-bound plane was a British national, possibly of Saudi origin, American officials have said.
UK intelligence played a central role in the operation that thwarted the plan to send a suicide attacker onto an aircraft with an underwear bomb, they added.
The undercover investigation reportedly involved MI5 and MI6, and the foiled plot ended with the explosive device being delivered to the FBI.
Its experts are examining it to see if it could have passed through security and been taken on to a plane.
The spy reportedly infiltrated a terror group in Yemen in a rare coup for Western agencies.
The individual is believed to have grown up in Europe where he was apparently radicalised.
He was subsequently "turned" and recruited by Saudi agents last year, it was claimed.
The reports were neither being confirmed nor denied in the UK.
The Obama administration had reportedly been under heavy pressure not to disclose the role of British authorities in the investigation.
US officials revealed publicly on Monday that al Qaeda in the Arabian Peninsula, known as AQAP, attempted to arm a suicide bomber with a non-metallic device.
It was an upgraded version of the failed underwear bomb which did not detonate on a Detroit-bound plane on Christmas Day 2009.
US officials had said the plot was foiled by the CIA and allied foreign intelligence services, without identifying the allies.
They added the operation was severely disrupted when leaks of some of the details began to emerge.
And they believed the operation could have continued at least another week or two if the leaks to the media had not occurred.
The director of National Intelligence has opened an "internal review" of US intelligence agencies to determine whether there were leaks of classified information related to operation, while the FBI is conducting a separate criminal investigation.
Defence Secretary Leon Panetta , a former CIA director, said: "I am fully in favour of a full and thorough investigation of this matter."
"When these leaks take place, I can't tell you how much they damage our ability to be able to pursue our intelligence efforts," Mr Panetta told reporters at the Pentagon.
A Department of Homeland Security official said that because the device was similar to the one in the failed 2009 attempt, security steps taken since "would have been able to prevent this device from bringing down an aeroplane".
Experts suggested airport body scanners, which use light doses of radiation to scan through a passenger's clothes, may have been able to detect an "anomaly" such as the device, which could then be further examined in a hands-on, pat down search.
However, the scanners have not been deployed in all airports across the US and are in very limited use elsewhere.
AQAP's master bomb-maker has previously been identified as Ibrahim Hassan al Asiri, a Saudi fugitive.
Officials believe the latest bomb could have been produced by al Asiri or one of his proteges.