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.
The UK intelligence services 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 "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.