Thousands using Britain as base for Islamic terrorism

The secret report, obtained by a Sunday newspaper, states that Britain will remain "a high-priority target" for international terrorists aligned with al-Qaida for the foreseeable future, and warns of a network of extremist cells in the UK, with the main concentrations in London, Birmingham and Luton.

The document – marked "restricted" – was reportedly drawn up by the intelligence branch of the Ministry of Defence, MI5 and Special Branch, and describes the threat from Islamist extremists as "diverse and widely distributed" with the number of terrorists in Britain "difficult to judge". However, it cites estimates from the Joint Terrorism Analysis Centre that there are "some thousands of extremists in the UK committed to supporting jihadi activities, either in the UK or abroad".

It goes on to state: "For the foreseeable future the UK will continue to be a high-priority target for international terrorists aligned with al Qaida.

"It will face a threat from British nationals, including Muslim converts, and UK-based foreign terrorists as well as terrorists planning attacks from abroad."

In 2006, the then head of the Security Service, Dame Eliza Manningham-Buller said that MI5 was aware of 30 major terrorist plots and had 1,600 people in 200 cells under surveillance. Last year, her successor Jonathan Evans said the number had grown to 2,000.

The new document paints a picture of the kinds of people caught up in extremist activity in the UK. It reads: "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 document also states that a number of extremists at large in the UK have been trained in terrorist camps overseas and have "some ability to construct improvised explosive devices, incorporating home-made explosives".

Last year Scotland On Sunday revealed that a hard core of 200 Islamic extremists with links to foreign terror groups had been identified by intelligence services as working north of the border. Senior intelligence insiders said the individuals concerned, many of whom were born and brought up in Scotland, posed a "significant" risk to public safety, and revealed that up to 1,000 Scottish Asians known to associate with radicals could be placed under surveillance, with their e-mails, mobile and landline calls all being monitored by GCHQ.

In October last year Mohammed Siddique became Scotland's first "home-grown" terrorist. The 21-year-old from Alva, Clackmannanshire, was found guilty of four separate terrorist offences, including providing material on how to make bombs. He was arrested at Glasgow airport in 2006, after several months of surveillance, as he attempted to board a flight to Pakistan. He was sentenced to eight years in prison.

A Home Office spokesman said it did not comment on leaked documents.