They have three things in common; they work for the NHS, are "clean skins" - people with no known extremist connections or previous convictions brought together for a specific operation - and they have not previously come up on the intelligence radar.

The first eight people arrested around the globe in this dramatically fast-moving counter terrorism operation allegedly formed a low-profile sleeper cell. Mobile phones seized from the Mercedes cars which failed to detonate in London's West End on Friday have helped officers unravel an international network operating quietly within a narrow corridor along the M6 motorway.

Seven doctors or medical students have been arrested in England, Scotland and Australia in connection with the attacks. All worked in NHS hospitals. Dr Hamish Meldrum, the British Medical Association chairman, said: "Like others we were shocked to hear of the recent attempted bombings.

"The news that members of a caring profession may be involved in these atrocities was even more appalling.

"Overseas doctors have made an invaluable contribution to the NHS over the years and it would be dreadful if the trust that exists between patients and doctors, whatever their background, was harmed by these events."

Working for the NHS provided the ideal cover. Although their car bombing attempts failed, the ramifications have already been felt with heightened security and a number of controlled explosions on suspicious packages and vehicles across the UK.

CIA sources said none of the main bombing suspects in British or Australian custody has shown up on US terrorist databases. The implication is that all are "clean skins".

The American lists of potential al Qaeda sympathisers includes tens of thousands of names gleaned from interrogations of prisoners at Guantanamo Bay and from the files of intelligence services from Jordan's Mahabarat internal security agency to Britain's MI5.

It also includes every potential terrorist name and alias known to the CIA, the FBI and the US National Security Agency, the biggest covert surveillance organisation in the world.

However, a US counter- terrorism source added yesterday that the bungled attacks on Glasgow airport and in London coincided with "indicators of increased movement of money and people from known al Qaeda camps in the ungoverned tribal territories of Waziristan on the Pakistani side of the Afghan border."

He refused to go into detail about a possible link between the stepped-up activity and the bomb threats in the UK, but said that White House reluctance to pressurise Pakistan into striking the training camps along the 1500-mile frontier had left al Qaeda "more secure than it has been at any time since before 9/11".

A spokesman for the US Department of Homeland Security said the weekend incidents in Britain had "heightened existing concerns", but denied that there had been knowledge of specific threats against the UK before the bombs were discovered.

"It is not just what happened in England and Scotland that has us watching closely. We have had concerns based on intelligence indicators for some time, " he added.

Bruce Hoffman, a counter-terrorism expert at Washington's Georgetown University, said he considered al Qaeda involvement in the UK incidents "likely".

The terror network had made a strategic investment in Britain in recent years, creating ties to an infrastructure of groups and individuals that are difficult to fit into an effective intelligence profile, he claimed.

"By drawing on a large pool of potential operatives, al Qaeda not only breaks any coherent attempt at profiling, but also demonstrates the diversity of its movement."

Mr Hoffman also criticised those who dismissed the failed plots as the work of amateurs. "They didn't work, but I think of all the al Qaeda plots we've seen, the sophistication of this one lies in its simplicity. The plotters used available materials. Where they tripped up was in the detonation of the devices. That's a trickier business."

A UK intelligence source agreed with Mr Hoffman. "The Provisional IRA usually tested their improvised bombs in some remote corner of rural Ulster before unleashing them.

"Arabs or Asians setting off gas canisters for practice in more heavily-populated Scotland might draw unwanted attention in the current climate.

"Their bombs were untested, luckily for everyone at the airport. The danger is, they might get it right next time."