By Jon Silverman BBC legal affairs correspondent

The radicalisation of young Muslims living in the UK, sometimes through contact with fundamentalist clerics, has caused the authorities concern for some years.

But the security services make a crucial distinction between those who are suspected of active links to the Bin Laden network and those who preach jihad in mosques and other forums but go no further than that.

In the first category is Sheikh Omah Abu Omar - more commonly known by the soubriquet, Abu Qatada.

Influence

Investigators in both the US and Spain have accused him of being a key envoy of Bin Laden in Europe.

He is a Jordanian citizen and came to the UK in 1993 as a political refugee after being sentenced to life imprisonment in Jordan for terrorism.

He denies any involvement in terrorism in the UK, but it is believed that he has influenced a number of young Muslims to become al-Qaeda conspirators.

Zacarias Moussaoui is under arrest in US

One is Zacarias Moussaoui, under arrest in the United States and strongly suspected of being the 20th hijacker on 11 September.

Another is Djamel Beghal, an Algerian arrested in Dubai just after 11 September and alleged to be the leader of a European terror network linked to Bin Laden.

Beghal attended the Finsbury Park mosque in north London, which has also radicalised hundreds of Muslims.

The Home Office denies that Britain has made it easier for clerics to gain asylum than other applicants and points out that it has never been complacent about the risks posed by those who use religion as a cover for fomenting terrorism.

Training camps

They refer to the case of Shafiq ur Rehman, a Pakistani cleric living in Oldham, who is facing deportation on national security grounds for allegedly recruiting British Muslims to fight for an independent Kashmir.

Legal moves against him began as long ago as 1998 and went all the way to the House of Lords.

Indeed, the Law Lords judgment in October 2001 - that someone accused of supporting terrorism abroad could be considered a threat to the UK's national security - has been incorporated into the anti-terrorism act which became law shortly before Christmas.

The British security service has tended to concentrate its monitoring on those young Muslims known to have gone abroad to fundamentalist training camps.

These are in the dozens, but many hundreds more who attend mosques in London and other cities may also represent a potential threat.

Neither the police nor M15 have the resources or manpower to keep them all under surveillance.