Underpants bomber Umar Abdulmutlallab was foiled in his plot to blow up a jumbo jet on Christmas Day because he had apparently soiled his bomb pants an American transport official has suggested.

British-educated Nigerian Abdulmutlallab stunned security experts by making it on board a flight wearing a bomb inside his pants in 2009.

But the device failed to detonate with the expected force and ended up injuring Abdulmutlallab, in the groin and anus.

John Pistole, head of the Transportation Security Administration, revealed the soiling in Abdulmutlallab's pants was what saved hundreds of lives that Christmas.

Describing how he nearly succeeded, Pistole said: "It was very close, the bomber had had the device with him for over two weeks.

"It was degraded. We're getting kind of personal now. Let's just say its efficacy was degraded."

Abdulmutlallab is serving life without parole for the terror plot.