The report says the UK will remain a target for terrorists for some time
More Islamist extremists are active in the UK than previously thought, a secret intelligence report seen by the Sunday Telegraph has suggested.
There are "some thousands" of militants in Britain, with extremist cells concentrated in London, Birmingham and Luton, the quoted report says.
The document also states the UK will remain "a high-priority target" for al-Qaeda for the foreseeable future.
The Home Office said resources had been invested to tackle the complex threat.
The paper says the document - marked "restricted" - was drawn up by the intelligence branch of the Ministry of Defence, MI5 and Special Branch.
Although the report states judging the number of militants is "difficult", it cites estimates from the Joint Terrorism Analysis Centre (JTAC) which claim there are "some thousands of extremists in the UK committed to supporting jihadi activities, either in the UK or abroad".
The document continues: "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 head of MI5, Jonathan Evans, warned in November last year that there were at least 2,000 people in the UK who posed a threat to national security because of their support for terrorism.
This was a rise of 400 since the previous year, he said.
And in April, Home Secretary Jacqui Smith said the terrorism threat facing the UK was "severe" and "growing".
But the latest document's figure of "some thousands" appears to suggest the numbers are greater than previous thought.
'Spreading extremist message'
The report also describes those involved in extremist activity in Britain.
"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," it says.
"The overwhelming majority of extremists are male, typically in the 18-30 age range.
We have always been clear - as has the director general of the Security Service - that there are many plots, individuals and groups under investigation
"The main extremist concentrations are in London, Birmingham, with significant extremist networks in the South East, notably Luton."
It also says the networks are principally engaged in "spreading their extremist message, training, fundraising and procuring non-lethal military equipment to support the jihads in Pakistan, Afghanistan and Iraq, and sending recruits to the conflicts".
It adds that "improvised explosive devices" intended to cause mass casualties "are the most likely form of attack in the UK".
A Home Office spokesman said it was aware the terrorist threat to the UK was "real" and "increasingly complex".
"The threat level is set by JTAC and remains at 'severe' as it has done for some time.
"We have always been clear - as has the director general of the Security Service - that there are many plots, individuals and groups under investigation."
"Significant resources" had been allocated to counter-terrorism policing and tackling radicalisation in the UK and abroad, he said.
He added that the government had a strategy for dealing with the immediate threat of attack, for bringing terrorists to justice and for taking action to prevent people becoming or supporting violent extremists.