Members of at least three terrorist cells linked to Osama bin Laden are at large in Britain. MI5 and MI6 are now searching for members of the Bin Laden network who may be connected with the suicide bombers behind last Tuesday's outrage.
The UK has been a major base and staging post for Mr bin Laden's operations. There are believed to be dozens of terrorists in Britain who have been associated with Mr bin Laden or trained at his camps.
The head of MI5, Stephen Lander, briefed Tony Blair on Mr bin Laden's British network at Downing Street yesterday.
One intelligence source said yesterday: "There is no reason why what happened in America couldn't happen in Britain or any European country. The terrorists are in place, and there is very little to stop them."
A Whitehall source added: "The problem is, these groups are amorphous and hard to identify until they commit a terrorist act. The degree of connection with Bin Laden varies. But they share his aims."
In two cases, alleged supporters of Mr bin Laden are being held in British jails facing extradition to the US. All are thought to be part of Mr bin Laden's loose network in the UK.
Khalid al Fawwaz was arrested in Britain with two other men, all wanted by the US authorities over suicide bombings on two US embassies in East Africa in 1998 which killed 228 people and wounded hundreds more. Both attacks were masterminded by Mr bin Laden.
British police, responding to requests from the US, arrested Mr al Fawwaz, 36, in September 1998. The FBI claims he is an influential member of Mr bin Laden's organisation Al-Qa'edah, which translates as "the Base". Mr al Fawwaz denies any involvement with terrorism.
In another case, Haydar Abu Doha, 37, was detained after Ahmed Ressam, another Algerian, was caught on his way to bomb Los Angeles airport shortly before 1 January 2000. In the boot of his car police found explosives and a business card belonging to Mr Doha.
A search of Mr Doha's home turned up several passports, fake identification and papers with chemical notations for explosives similar to those found in Mr Ressam's car. Mr Doha is now in Brixton prison fighting extradition. He denies any wrongdoing.
Mr bin Laden and his associate networks are said to span 34 countries, planning a dozen terrorist actions at any one time. On Friday an alleged member was picked up in Brussels. Last spring European police conducted a series of co-ordinated raids against Islamic extremists, arresting 18 suspects. Four were detained at Heathrow.
A month later a raid in Milan yielded tapes and documents showing Bin Laden connections in Britain, France and Germany. Two tapes feature the voices of two suspected terrorists watching videos of Chechen Mujahedin in action against Russian soldiers. They discuss the arrest of Mr Abu Doha in London.