LONDON - SECRET enclaves of Al-Qaeda extremists based in London, Birmingham and Luton are planning mass-casualty attacks in Britain, according to a leaked government intelligence report.

The document, drawn up by the intelligence branch of the Ministry of Defence, MI5 and Special Branch, states that 'some thousands' of extremists are active in the United Kingdom, according to the Sunday Telegraph.

It estimates that most of them were born in the United Kingdom and are aged between 18 and 30 and many could have been trained in overseas terrorist camps.

A year ago, MI5 director-general Jonathan Evans said in a speech that his organisation had found that there were at least 2,000 individuals who posed a threat to national security and public safety.

Since 2001, over 1,200 terrorist suspects have been arrested, over 140 have been charged and more than 45 have been convicted of terrorism offences, according to Home Office figures. It is also estimated that there are some 200 terrorist networks functioning in Britain today which are involved in at least 30 plots.

But this latest security assessment appears to suggest that the number of individuals who pose a threat is even higher.

The report continues: 'The majority of extremists are British nationals of South Asian, mainly Pakistani, origin but there are also extremists from North and East Africa, Iraq and the Middle East, and a number of converts. The overwhelming majority of extremists are male, typically in the 18-30 age range.'

According to the report, they are concentrated in London and Birmingham, with a significant network in Luton. They are engaged mainly in 'spreading their extremist message, training, fund-raising and procuring non-lethal military equipment to support the jihads in Pakistan, Afghanistan and Iraq, and sending recruits to the conflicts'.

The report adds: 'UK-based extremists, either under the direction of Al-Qaeda, or inspired by Al-Qaeda's ideology of global jihad, have also engaged in attack planning in the UK.'

To deal with the growing number of terrorists, MI5 now has nine regional offices and has almost doubled its staff numbers from 1,800 in 2001 to 3,500 today.

There are around 1.5 million Muslims in Britain, a million of whom live in London.