Prosecutors said they thought Saajid Badat had backed out of an alleged plot with Richard Reid, who was subdued by passengers when he attempted to detonate a bomb aboard an American Airlines flight from Paris to Miami on 22 December 2001.
Judge Adrian Fulford, sentencing him on Friday, commented that "turning away from crime in circumstances such as these constitutes a powerful mitigating factor. It can take considerable courage to plead guilty to offences of this kind."
First major conviction
Badat's guilty plea in February was the first major conviction for a terrorist plot in Britain since the 11 September 2001 attacks in the United States.
British convicts are typically eligible for parole after serving two-thirds of their sentence, so Badat could be free in a little more than eight years.
Fulford said Badat had been part of a plot to commit a "wicked and inhuman crime" that would have killed hundreds of people.
"Sitting in the civilised and muted surroundings of the Old Bailey, it is easy to forget exactly what you planned," he told Badat.
Change of heart
But the judge said he thought the would-be terrorist had had a genuine change of heart. He said he balanced the need for strong deterrents in terrorism cases with Badat's evident remorse.
Fulford said he hoped the sentence would send a message to others considering terrorism that a decision to turn away from violence would benefit them in court.
Had Badat been convicted at trial without pleading guilty, the judge said, he would have recommended a sentence of at least 50 years.