Underwear bomber's plot was foiled by British agent

The undercover agent who foiled an an Al-Qa'ida plot to destroy an airliner by posing as a potential suicide bomber is British, it was reported in the US last night.

The operative, who is of Saudi origin but holds a UK passport, is said to have been involved with a group of Islamic extremists but was recruited to help fight against terrorism after coming to the attention of intelligence officers.

His British nationality played a key part in his appeal to those plotting the attack, according to CNN, because he would find it easier to arrange travel to the US.

He is understood to have been sent to Yemen at the behest of Saudi counter-terrorism officers, though Reuters reported last night that British intelligence played a "central role" in thwarting the attack.

The unnamed man's genuine background in extremism was important because the terrorist network pay extremely close scrutiny to the reliability of those they recruit. An anonymous security source, who had been briefed by Saudi agents, told CNN that the double agent informed the authorities in the Middle Eastern kingdom about the plot, who then passed the details on to the US.

After being taught how to use the weapon at a training camp, the agent and a fellow informant were extracted from Yemen with the bomb and flown to the Saudi capital, Riyadh.

The FBI has since been studying the device at its lab at Quantico, Virginia, and US officials say it bears the hallmarks of fugitive Saudi militant Ibrahim Hassan al-Asiri, suspected of being the chief bomb-maker working with Al-Qa'ida in the Arabian Peninsula, as the Yemeni division is known.

The terrorist group is believed to have been planning the attack with an almost undetectable, non-metallic device to coincide with the first anniversary of the death of Osama bin Laden.

The device was an upgraded version of the underwear bomb that was carried onto a Detroit-bound plane on Christmas Day 2009.