An al-Qaeda supergrass was brought to the Old Bailey amid heavy security today to give evidence in the trial of seven British men accused of plotting to bomb the UK.

A helicopter circled overhead and police outriders drove alongside as a white van carrying Mohammed Babar, 31, arrived at the court in central London.

Armed officers stood guard as Babar, a former member of al-Qaeda who has been convicted on terrorism charges in New York, started to give his evidence. He claimed that he had met the defendants when he moved to Pakistan after the September 11 attacks.

Speaking with a soft American accent, wearing glasses but not handcuffs, Babar told the court of his journey to radical Islamism.

He said that he had become increasingly politicised after the Gulf War in 1991 and that he tried become a jihadi in Chechnya and the Palestinian Territories during the 1990s, reading the website of Abu Hamza, the former imam of Finsbury Park Mosque who was jailed for hate crimes in January.

Babar, a former parking attendant, said he also searched for the right extremist organisation to join, finally settling on Al-Muhajiroun, a group formally banned in the UK and led by Omar Bakri Mohammed, a radical cleric who left the country after the London bombings last July. But it was 9/11 that finally galvanised Mr Babar to go abroad.

He told the court that he decided to take up arms against the US, where he had lived since he was two, even though his mother was in the World Trade Center when it was hit by hijacked aircraft.

"My mother was involved in it. My mother worked in the World Trade Center when the first World Trade Center was hit. She was in the building but she survived," he said.

Babar said he flew first to London, to make contact with other extremists, who could help him find his way to Pakistan and Afghanistan. He was taken by al-Muhajiron members to a house in Southall, west London, given £300 and brought to a protest outside the Pakistani High Commission.

Asked by David Waters, QC, prosecuting, whether he knew that by heading to Pakistan he was likely to end up fighting American soldiers, Babar replied: "Yes".

Babar is expected to be the star witness in the trial of the seven men accused of planning to bombing campaign in the UK. On the first day of the trial, Mr Waters told the court that Babar had met and trained some of the defendants and tested explosives with them in the mountains of northern Pakistan.

Mr Waters told the court that two of the charges Babar pleaded guilty to in New York applied to what American prosecutors described as "the British bomb plot".

Yesterday the court was told how the group considered targetting nightclubs, electricity and gas pipelines and the Bluewater shopping centre in Kent.

Babar said he met the defendants among a group of as many as 20 "brothers" who had flown out to Pakistan from London and the south east to take arms against America.

"Those brothers who came basically from England and were in Pakistan at the time after 9/11, 15 or 20 of us came to Pakistan for the jiahd," he said. "The majority had Pakistani ancestry and were from the UK, basically from Crawley and London."

Asked by the court to name the seven men on trial, Babar handed Mr Waters a list of their aliases.

The accused are: Omar Khyam, 24, Waheed Mahmood, 34, Shujah Mahmood, 19, and Jawad Akbar, 22, all from Crawley, West Sussex; Anthony Garcia (also known as Rahman Adam), 23, of Ilford, East London; Nabeel Hussain, 20, of Horley, Surrey, and Salahuddin Amin, 31, from Luton, Bedfordshire. They deny the charge of conspiring to cause explosions.

Mr Khyam, Mr Garcia and Mr Hussain also deny a charge under the Terrorism Act of possessing 600kg of ammonium nitrate fertiliser, which can be used to make bombs. Mr Khyam and Shujah Mahmood further deny possessing aluminium powder for terrorism. The men face life imprisonment if convicted.

The trial continues.

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