Hamza is said to have been so convinced by a British undercover investigator posing as an extremist website operator that he allegedly sent him several secret propaganda films designed to attract new recruits. The videos were used, say investigators, to convince British Muslims to undergo jihad training at camps in Afghanistan and Bosnia.

The tapes and e-mails were obtained by Glen Jenvey, a 38-year-old freelance counterintelligence investigator from Wiltshire, over a period of more than a year. As the evidence flowed in, Jenvey forwarded it to the FBI, which is now building a case to extradite Hamza to America.

Last week Scotland Yard confirmed that anti-terrorist branch officers had taken a statement from Jenvey and sent a copy to the FBI.

The evidence is being marshalled by US government prosecutors in New York, where Hamza is part of a grand jury investigation into a plot to provide weapons training to American mujaheddin on a cattle ranch in Bly, Oregon.

If the grand jury charges Hamza with conspiracy to provide material support to terrorists, which carries a 25-year jail sentence, sources close to the case say they will press for his extradition to America.

According to statements given to the anti-terrorist branch in March, Jenvey set up an internet site called islamic-news.co.uk in 2002 using the fictitious name Pervez Khan. He published news items on the site from Kashmiri extremist groups and other hardline Islamic propaganda.

Once established, he sent the material to Hamza’s website — supportersofshariah.org, which is now shut down. “He was so pleased with this he decided to put a link to my site from his site. That was his first big mistake,” said Jenvey last week.

As a consequence, Jenvey was able to monitor everyone who logged on to Hamza’s website, a facility he passed on to the FBI so that it could also monitor Hamza’s activities.

He then decided to take the operation a step further. “By now I was getting close to Hamza. He trusted me,” said Jenvey. “We had been e-mailing each other a lot and I had been passing the e-mails to the FBI. We also started to speak on the phone.

“I started to suggest I could help him recruit people for his jihad. He became very excited by this. He would burble prayers down the phone in an almost demented fashion. I thought he must be a bit mad.

“He said he would send me some material to help me win supporters and prepare them for jihad. He said he had some special tapes but that they were somewhere secret and he did not keep them in the mosque. He said they would be sent to me.”

Less than a month later a package arrived at Jenvey’s home. Inside were 20 audio tapes. A second package arrived a week later with six two-hour video tapes. Jenvey was shocked by what he saw. One tape starts by showing a training camp in Bosnia and scenes of urban combat training. Jihad anthems play in the background and a voice in English says: “Make ready to continue to terrorise the enemy of Allah.”

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