The 2009 "underpants bomb" plot failed because the terrorist had been wearing his explosive-laden undergarments for more than two weeks and soiled the explosives, a senior US official said.

Umar Abdulmutallab sent shockwaves through US intelligence when he successfully smuggled a bomb onto a Detroit-bound airliner on Christmas day three years ago. The British-educated Nigerian was able to light the bomb but it failed to explode, causing minor burns to the would-be bomber but sparing his fellow passengers.

John Pistole, the head of the Transportation Security Administration, said the bomb did not detonate because Abdulmutallab had been wearing the same underwear for more than two weeks. Asked whether the bomb's fuse had become "damp" from two weeks of wear, Mr Pistole said: "Let's say it was degraded. We're getting kind of personal now."