Since then his name has occasionally surfaced during terror investigations abroad but he was nowhere near the top of Britain’s wanted list. Today he sits in an Islamabad prison waiting to be interviewed by British officials anxious to discover if he played any part in the London attacks.

His arrest in April barely rated a mention at the time, even though Pakistani authorities were accusing of him being linked to some of al-Qaeda’s most notorious figures.

They include its so-called operational commander, Abu Faraj al-Libbi, who is in US custody.

Al-Libbi allegedly ran al- Qaeda’s sleeper cells overseas and sanctioned a suicide attack on London.

Over the weekend enough connections were established between this British cell and radical groups abroad to show that the London bombers were part of a widespread and well- organised network and not the “clean skins” as was originally stated. The links between those who “talent-spotted” the four and those who persuaded them to take part in violence are tortuously complicated.

But much of it, say British security sources, leads back to Pakistan.

It was there that the bombers met militants linked with al-Qaeda and other outlawed groups.

This is why Sir Ian Blair, the Commissioner of the Metropolitan Police, is confidently asserting that al-Qaeda will be found to be responsible at the end of the murder hunt.

For the moment detectives are still piecing together how the four men became enmeshed in a network that has produced at least 13 British-born suicide bombers, probably more.

Investigators are looking at the activities of what they are calling the “forgotten men”, the likes of Siddiqui, whom the authorities admit losing touch with over the past decade.

Some may have drifted home in obscurity. Others died in wars, including in Afghanistan. But there is a generation of young Britons responsible for helping to recruit their own countrymen.

Security sources in Islamabad have told The Times that earlier this year Siddiqui, a college dropout, met one of the British bombers — Shehzad Tanweer — who quit his university course in Leeds to go to Pakistan.

It is the question of to whom else Siddiqui may have introduced his visitors during their time in Pakistan that disturbs the Yard.

His name first came to light after it was disclosed that his best friend at Cranford Community College was Asif Hanif, who died in April 2003 in a suicide attack on an Israeli nightclub.

Yesterday Maariv, an Israeli paper, claimed that another of the British bombers, Mohammad Sidique Khan, helped to organise the terror attack on a Tel Aviv that was carried out by two Britons. The Times has learnt from Pakistani security sources that while in Pakistan some of the Tube bombers met leading figures from an outlawed terror group called Jaish-e-Mohammed, which has been actively recruiting Britons from universities and colleges since the early 1990s.

They and another banned group — Lashkar-e-Taiba — collect more than £5 million each a year from British mosques, even though both are proscribed organisations.

Most donors believe that their funds are going to charities and educational projects associated with the conflict in Kashmir.

A report by the Brussels-based International Crisis Group, says that Britain remains one of the main donors but cautions that the diversion of funds from the humanitarian projects to terror cells has become a normal practice. “It is difficult to separate finances for terror from those for charity,” the report says.

In the past Jaish has boasted of many British Muslim volunteers in its ranks. Among them is Ahmed Omar Sheikh, a former LSE student and public schoolboy from Woodford, northeast London, who abandoned his studies in 1993.

He is now facing a death sentence in Pakistan for the abduction and murder of Daniel Pearl, the Wall Street Journal reporter.

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