Saajid Badat, 33, appearing on video in Manhattan federal court, is expected to testify about another man, Feroz Abbasi, who prosecutors claim Abu Hamza sent from London to Afghanistan to train with al Qaeda.
According to prosecutors, Badat will testify that he witnessed Abbasi meeting with al Qaeda leaders and discussed carrying out attacks, bolstering the government's argument that Abu Hamza sent Abbasi there to fight alongside al Qaeda.
Abu Hamza is also accused of assisting militants who took 16 tourists hostage in Yemen in 1998 in an operation that left three of the tourists dead and of trying to set up a jihadist training camp in rural Oregon.
The Egyptian-born Abu Hamza, 56, lost an eye and his hands in Afghanistan in the 1980s. He became known for his sermons at the Finsbury Park mosque in north London before being convicted in Britain of inciting his followers to violence; he was extradited to the United States in 2012.
Abu Hamza's defense lawyers have argued that he used inflammatory language but did not take part in any plots.
Badat said he had never met Abu Hamza in person but saw him preach twice at the Finsbury mosque in 1997, though he could not recall much detail about the sermons.
During the morning session, Badat, who appeared on a monitor set up against one wall of the courtroom,- said testifying in such trials is "almost like getting my revenge" on individuals like Khalid Sheikh Mohammed, the alleged mastermind of the Sept. 11, 2001, attacks in the United States, who should have known better than to influence young men to turn to violence.
Badat plotted with Briton Richard Reid to blow up airplanes using explosives hidden in their shoes. He backed out at the last minute after his parents, who suspected he might have been radicalized, intervened and told him they hoped he did not belong to a "sleeper" cell, according to his testimony.
Reid, meanwhile, unsuccessfully attempted to detonate his bomb aboard an airplane in December 2001 and is currently serving a life sentence in the United States.
"In 2001, I was tasked by the leadership of al Qaeda, a terrorism organization, to take on board an aircraft an explosive and detonate it," Badat said at the start of his testimony on Monday.
Badat is known in British parlance as a "supergrass," a prolific government informant. His testimony has been used in several other U.S. terrorism cases, including the trial of Osama bin Laden's son-in-law Suleiman Abu Ghaith, who was convicted last month in the same courthouse where Abu Hamza is on trial.
Badat has refused to travel to the United States for fear he would be arrested on pending federal charges related to the shoe bomb plot. He pleaded guilty in the United Kingdom, agreed to cooperate with authorities and served five years in prison.
His testimony on Monday mirrored the evidence he delivered in Abu Ghaith's trial, as he gave jurors a glimpse of life inside al Qaeda in the years before and after the Sept. 11 attacks, which killed nearly 3,000 people.
While in Afghanistan, he said, he met with senior al Qaeda figures about the shoe bomb plot, including bin Laden; Khalid Sheikh Mohammed; Ayman al-Zawahiri, al Qaeda's current leader; and al Qaeda's former military chief, Mohammed Atef.