Prosecutors said 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 Dec. 22, 2001.
''Turning away from crime in circumstances such as these constitutes a powerful mitigating factor,'' judge Adrian Fulford said. ''It can take considerable courage to plead guilty to offenses of this kind.''
Badat's guilty plea in February was the first major conviction for a terrorist plot in Britain since the Sept. 11, 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 over 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 civilized and muted surroundings of the Old Bailey (courthouse), it is easy to forget exactly what you planned,'' he told Badat.
But the judge said he believed 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.
In letters to his parents written before the bomb plot and read in court Friday, Badat said he was disillusioned with Britain.
He wrote: ''I have a sincere desire to sell my soul to Allah in return for paradise.''
Prosecutors said the letter was found along with explosives at his home.
Prosecutor Richard Horwell said Badat had confessed as soon as he was apprehended in November 2003, telling officers as they drove to the police station: ''I was asked to do a shoe-bombing like Richard Reid.''
He told the police about a green suitcase in his bedroom which contained a fuse and detonator, and another suitcase which contained explosives inside a sock.
He reportedly told officers he didn't know how to dispose of the items, and added: ''An Arab gave me these things in Afghanistan.''
Defense lawyer Michael Mansfield said the tale of how a ''conscientious, hardworking student'' came to consider mass murder ''is a story of our times.''
Badat's Muslim faith ''in one sense took him to the brink, the very brink of disaster, and at the same time it was same faith that pulled him back,'' Mansfield said.
Mansfield said Badat felt deep remorse and wanted to urge anyone else considering terrorist acts to ''have the courage to turn back and save lives.''
Badat, 25, of Gloucester, England, had been accused of conspiring with Reid, who was convicted in the United States, and with a Belgian man to make the explosive device.
Reid was arrested after trying to detonate his bomb aboard the Paris-Miami flight on Dec. 22, 2001. He was sentenced to life in prison after pleading guilty to U.S. charges.
Horwell said before Badat's plea that he had booked a ticket to fly from Manchester, England, to Amsterdam, Netherlands, in preparation for an onward flight to America on which he planned to detonate his bomb.
''But he did not take that flight. We accept by then he had withdrawn from the conspiracy, which by then he had been party to for an appreciable period of time,'' Horwell said.
He said Badat sent an e-mail on Dec. 14, 2001, indicating he might withdraw from the plot.
Horwell said Badat kept the device at his home in Gloucester, but had separated the fuse and detonator from the plastic explosive.
Prosecutors had said Badat received training both in Afghanistan and Pakistan, and that while in Afghanistan he was given an explosive device designed to evade airport security and destroy an aircraft in flight.
Badat returned to Britain on Dec. 10, 2001, with the device in his possession. The detonating cords on Reid's device matched the cord on Badat's bomb, prosecutors said.
In October, a U.S. grand jury in Boston, Massachusetts, charged Badat with attempted murder, trying to destroy an aircraft and other counts related to the alleged conspiracy.
Police said Belgian telephone cards found on Reid were used by Badat to get in touch with Reid's terrorist contact Nizar Trabelsi, who is now in jail in Belgium.
Trabelsi, a Tunisian who played professional soccer for several German teams, was sentenced to the maximum 10 years in prison in 2003. He had admitted planning to drive a car bomb into the canteen of a Belgian air base where U.S. nuclear weapons are believed to be stored.
Badat's parents reportedly had emigrated from Malawi in 1960s and settled in Gloucester, where Badat was born. He pleaded guilty to conspiring between Jan. 1, 1999, and Nov. 28, 2003, to place a device on an aircraft in service.
