Saajid Muhammad Badat was convicted of planning to blow up a transatlantic airline in 2005 in a plot which also resulted in the jailing of Richard Reid, the "shoe bomber".
Badat was released from prison in the UK in March 2010 after having two years taken off his sentence for agreeing to turn "supergrass" against other al-Qaeda terrorists.
Details of the secret deal were made public only after his involvement in the trial of Adis Medunjanin, a suspected terrorist accused of plotting a suicide bomb attack on the New York subway, was revealed earlier this week.
On Thursday, a short extract of video evidence was played to the US Federal Court in Brooklyn in which Badat disclosed that he did not know who Medunjanin was, raising further questions about whether his testimony was crucial enough to warrant his early release.
Prosecutors believe Badat's co-operation is useful because of his apparent strong links with the al-Qaeda leadership.
On Thursday it was disclosed that he met Osama bin Laden on more than one occasion. Badat was charged in 2003 with conspiracy to destroy an aircraft after he plotted to detonate a bomb on a transatlantic flight from Amsterdam.
He explained that he had run errands for bin Laden while in Afghanistan between 1999 and 2001when he was aged just 19 and had "direct personal interactions" with the terror chief.
Asked why he was in Afghanistan, Badat replied: "In order to acquire military training for a jihad, a violent jihad….to fight with the oppressors of Muslims."
He explained that he had attended training camps Kabul, Jalalabad and Kandahar where he learned to operate weapons including machine guns and rocket propelled grenades.
In the video Badat wore a grey suit, white shirt and blue tie. But his appearance has altered dramatically since his arrest in 2003. He has trimmed his long beard, no longer wears glasses and is bald.
He is currently believed to be living in Gloucester and police fear his public testimony in the US could put Badat in danger.
Badat was told in the video that if he does not tell the truth in his testimony he will face further charges in the UK and US.
Badat pleaded guilty in the UK and was also charged in the US, but has only been convicted in Britain.
Scotland Yard tried to suppress his evidence to the US over concerns that he could be in danger from any al-Qaeda operatives he testified against.
His evidence is scheduled to continue on Monday.
