The UK's security services fear that Al Qaeda terrorists have trained a team of British-based radicals.

They say at least 12 would-be suicide bombers with British credentials have been arrested or killed in recent years, but many more are waiting to be activated.

Last week's detention of Sajid Badat has brought home to security chiefs just how close the country has been to a suicide attack wholly operated and sponsored from within the UK.

Buckingham Palace, London's Heathrow airport and major shopping centres are still considered likely targets for bombers in the run up to Christmas.

A massive effort is now underway to track every UK resident known to have visited Afghanistan, where Al Qaeda ran a string of training camps like Khalden and Darunta, in the past five years.

Badat was arrested at his parents' home in Gloucester, where police also found explosives. Members of Scotland Yard's anti-terrorist squad have subsequently given contradictory briefings on whether they also recovered heels of shoes in which Badat had planned to secrete a kilogram of PETN explosive.

The key to Badat's arrest is the extraordinarily detailed surveillance to which he was subjected ever since returning from Afghanistan and Pakistan more than a year ago.

In the eyes of the security services the huge resources devoted to following him have been more than justified by the discovery of the PETN explosive.

Yet Badat is the only the tip of the iceberg. As far as the security services are concerned at least another 40 are still at large and they are quite separate from another 12, including Badat, who have been neutralised.

The dirty dozen, after taking Badat into account, include Mohammed Bilal, 24, from Birmingham, who blew himself up at an army barracks in Srinagar in 2000; Asif Mohammed Hanif, 21, from London; Omar Khan Sharif, 27, from Derby, who died last April after attacking a Tel Aviv night club; Lamine Maroni, 32, an Algerian asylum seeker from Sheffield, who was convicted by a German court last March of plotting a suicide attack in Strasbourg; Wail al Dhaleai, a 22 year old Yemeni asylum seeker also from Sheffield who died in Iraq earlier this month after a suicide attack on US troops near Baghdad; Richard Reid, 29, the shoe-bomber from South London, who was jailed in the US earlier this year after he tried to ignite the explosives in his shoe on a flight from Paris to Miami.

In addition to these six, security sources have named four others with strong UK connections. They include Leicester-based Jamal Bheghal, an Al Qaeda recruiter who is now in French custody; Moroccan-born Zacharia Moussawi, the so-called 20th hijacker, who attended a South London mosque in Brixton along with two white Frenchmen, David and Jerome Caurtailler, who are under arrest back home in France.

Liverpool-based Mohammed Rashid Ohwali was another would-be suicide bomber who jumped out of his lorry shortly before it crashed into the US embassy in Nairobi.

Badat and the others all have in common a recruitment pattern that starts in a British mosque, then continues to a clearinghouse in Peshawar, known as the Martyrs House, and on to the Khalden or Darunta training camps across the border in Afghanistan.

Security experts' concerns that graduates of Darunta and Khalden are still at large in the UK lie behind separate warnings from Home Secretary David Blunkett, MI5 Director-General Elizabeth Manningham-Buller and Scotland Yard chief Sir John Stevens that it is not an issue of 'if' but 'when' an Al Qaeda attack is mounted in the UK.