Secret enclaves of al-Qaeda extremists based in London, Birmingham and Luton are planning mass-casualty attacks in Britain

The document, which was 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 UK. They are predominantly UK-born and aged between 18 and 30, and many are believed to have been trained in overseas terrorist camps.

Under the heading "International Terrorism", the report, which is marked "restricted" states: "For the foreseeable future the UK will continue to be a high-priority target for international terrorists aligned with al-Qaeda. It will face a threat from British nationals, including Muslim converts, and UK-based foreign terrorists, as well as terrorists planning attacks from abroad."

The report states that the threat from the Islamist extremist community in the UK is "diverse and widely distributed" but adds that the numbers of terrorist in Britain is "difficult to judge".

The document does state, however, that the Joint Terrorism Analysis Centre, which is based in MI5's headquarters at Thames House in London, estimates that there are "some thousands of extremists in the UK committed to supporting Jihadi activities, either in the UK or abroad".

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

Since 2001, over 1,200 terrorist suspect 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 who are involved in at least 30 plots.

But this latest security assessment appears to suggest that the number of individuals who now pose a threat to the UK 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.

"The main extremist concentrations are in London, Birmingham, with significant extremist networks in the South East, notably Luton. Extremist networks are principally engaged 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.

"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."

Although the document specifically names London, Birmingham and south east England as areas of extremist activity, MI5 believe that the threat posed by Islamist extremists comes from across the UK. In an attempt 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. There are 150,000 Muslims in Birmingham and a further 27,000 in Luton. There are also an estimated 10,000 Afro-Caribbean Muslims or white converts.

Some of the terrorists involved in the plot to bring down airliners using liquid bombs came from London, where a bomb factory had been established.

Birmingham, one of the centres of Islamic radicalisation in Britain, was where a plot was formed to kidnap and behead a British soldier.

The plot was lead by Parviz Khan, an unemployed charity worker who formed a terrorist cell in the city. The extremists planned to video the execution and release the film on the internet.

Luton has a growing Muslim population and has been a hot-bed of radical activity. The extremist group al-Muhajiroun has also been very active in the town. The 7/7 bombers assembled in Luton, before travelling to London to carry out their attacks.

The document also reveals that many of the terror networks operating in the UK include extremists who have been trained in terrorist camps overseas and have "some ability to construct improvised explosive devices, incorporating home-made explosives".

It adds: "The availability of training/guidance and the necessary components to build improvised explosive devices (IED), allied with extremists' known targeting preferences, mean that IED attacks against crowded places, intended to cause mass casualties, are the most likely form of attack in the UK."

It is also made clear in the report that al-Qaeda cells are planning further attacks in UK with the so-called Government Security Zone (GSZ), an area which covers the Houses of Parliament, Whitehall, Buckingham and St James' Palaces, as a possible target. The threat level in the GSZ is described in the report as "severe".

Security officials are convinced that UK-based al-Qaeda cells will attempt to carry out another "spectacular" inside the UK with major transport termini, such as airports and train stations, being the most likely targets.

Patrick Mercer, the Tory MP for Newark, said al-Qaeda now had support in large parts of the country, especially around Luton which was the spot where the 7/7 terrorists assembled before travelling to London to mount the Tube bombings.

He added: "We know that subversion and support for al-Qaeda is taking place in campuses and prisons all over the UK. The fact that we have not been attacked for over two years should not be taken by anyone as evidence that the threat has gone away, in fact it is just the contrary."