A double agent allegedly recruited by MI5 and MI6 was a British passport holder, according to American media reports.
he man is said to be from the Middle East, but to have grown up in Europe and become a British citizen.
UK and Saudi agents were involved in penetrating a plot by al-Qaeda in the Arabian Peninsula to detonate a sophisticated underwear bomb aboard an American jetliner, NBC said, citing unnamed security sources. The alleged bomber has not been identified publicly.
Officials said the bomb - a refined version of an "underwear bomb" used in two previous failed plots - was driven from Yemen to Saudi Arabia and was being examined by US experts at FBI labs in Virginia.
British officials were reportedly left "slack-jawed" by the leaks and detailed reported in the US media, the Times reported, quoting Whitehall officials.
MI5 recruited the agent and MI6 coordinated with Saudi intelligence, who had already infiltrated the Yemen-based group, the Daily Telegraph said.
MI5 and MI6 may be reluctant to take credit for the operation if it is linked to the US drone attack on Yemen that killed a militant alleged to have been involved in the 2000 USS Cole bombing, the Guardian said.
"The revelation is politically and legally awkward for MI6 and MI5 whose agents, unlike American ones, are banned from missions that lead to assassinations," the Guardian reported.
Foreign Affairs correspondent Jonathan Rugman writes:
Secret agents risk their lives working for British Intelligence and despite advances in surveillance technology, these human sources of information remain at the heart of the Secret Intelligence Service's work.
If the foiling of a second "underpants" bomb plot involved a double agent who was British and recruited in the UK as has been reported, then MI5 and MI6 will be angry that so much has been disclosed. This is partly because both organisations are steeped in secrecy, partly because it erodes trust with partner intelligence agencies around the world, and partly because it will put other agents at risk on the field. After all, al-Qaeda will be on its guard against further infiltration even more so than it was already.
It is unclear whether this agent was being run by the British, the Americans or the Saudis. The distinction could be important, because agents run by MI6 are not allowed to be involved in missions which lead to assassinations. But in a world of secrecy which depends almost entirely on inter-agency cooperation, the agent could have been transferred to non-British control.
There have been at least two US drone strikes against al-Qaeda in Yemen since the agent brought his undetonated bomb back to his handlers. The Americans say that among the dead is Fahd al-Quso, wanted for his alleged involvement in the bombing of the USS Cole in 2000. It seems highly likely that the double agent's information led to al-Quso's death.
Bomber a European
Quoting sources briefed by Saudi counter-terrorism officials, US television networks said the would-be bomber grew up in Europe where he was apparently radicalised.
He was subsequently "turned" and recruited by Saudi agents last year, they said.
The reports were neither confirmed nor denied in the UK.
The device is similar to a bomb sewn into the underwear of a man who tried and failed to set it off over Detroit on Christmas Day 2009.
Al-Qaeda in the Arabian Peninsula is believed to have been planning the spectacular attack, with a nearly-undetectable device, around the anniversary of the death of Osama bin Laden.