An undercover agent who foiled an al Qaida plot to blow up an airliner with an underwear bomb is a British national, it has been reported.
UK intelligence were said to have been "heavily involved" in recruiting the spy who infiltrated a terror group in Yemen in a rare coup for Western agencies.
Quoting sources briefed by Saudi counter-terrorism officials, US television networks said the individual 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 being confirmed nor denied in the UK.
Al Qaida 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. But the individual chosen to carry out the attack was an informant who handed over the bomb.
US officials said the would-be bomber was supposed to board a plane to the US and detonate the device inside the country. The CIA became aware of the plot last month.
FBI experts are examining the bomb to see if it could have passed through security and been taken on to a plane. Officials said it was an upgrade on one which failed to detonate on a flight over Detroit on Christmas Day 2009. Both were designed to be carried in a passenger's underwear, but the latest version contained a more advanced detonation system.
The 2009 bomb, and an incident the following year when terrorists smuggled bombs on to cargo jets, were believed to have been the work of al Qaida bomb-maker Ibrahim Hassan al-Asiri in Yemen. Officials believe the latest bomb - details of which became public on Monday night - could have been produced by al-Asiri or one of his proteges.
Downing Street declined to comment on the case. David Cameron's official spokesman told reporters at a regular daily briefing: "I understand there is an investigation under way, being led by the Americans. It is clearly a matter for the US authorities. Clearly, we think that sensitive information should be protected."