The threat of homegrown terrorists attacking Britain is greater now than any time since the September 11 attacks in the US, it has been reported.
More than 2,000 British-based Islamic terrorists are believed to be plotting attacks, according to a leaked Government threat assessment.
"The scale of al-Qaida's ambitions towards attacking the UK and the number of UK extremists prepared to participate in attacks are even greater than we previously judged," the Daily Telegraph quoted the document as saying.
The newspaper said the document was being circulated between the Home Office, Defence Department, M15 intelligence agency and Scotland Yard's Anti-Terrorist Branch.
The Home Office declined to comment on the report, but said in a statement that security arrangements are under constant review.
"As (MI5 Director-General) Eliza Manningham-Buller has stated publicly, the threat of terrorism in the UK is very real and includes the intent to kill people and damage our economy," the statement said.
Manningham-Buller said in November that 1,600 people were suspected of involvement in terrorist plots against British targets.
Security officials say they have foiled at least six plots since the July 7 attacks. Officials expect the number of plots to increase this year.
The report also said Afghanistan is expected to increasingly become a magnet for Islamic extremists seeking to fight Western military forces.
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NATO is struggling to persuade its members to send more troops to southern Afghanistan where the alliance is fighting a growing Taliban insurgency that left 4,000 dead last year.
