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 foiled plot ended with the explosive device being delivered to the FBI, which is examining it.
The investigation involved MI5 and MI6, officials added.
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 was carried onto 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.
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 airplane".
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.
"I'm convinced that Asiri is behind this. He is an evil genius when it comes to bomb- making," House of Representatives homeland security committee chairman Peter King said on Fox News.