The threat of homegrown terrorists attacking Britain is greater now than at any time since the September 11 attacks in the US, according to a leaked document.

More than 2000 British-based Islamic terrorists are believed to be plotting attacks, according to a government threat assessment prepared this month and seen by The Sunday Telegraph.

Click here to read the original article on the Telegraph Web site.

The number is far greater than was previously thought by the security services.

"The scale of Al Qaeda'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 newspaper quoted the document as saying.

"We still believe that AQ (Al Qaeda) will continue to seek opportunities for mass-casualty attacks against soft targets and key infrastructure. These attacks are likely to involve the use of suicide operatives."

The document was circulated between the Home Office, Defence Ministry, MI5 intelligence agency and Scotland Yard's Anti-Terrorist Branch.

MI5 believed that soft targets, such as the transport system and economic targets such as London and major business centres such as Canary Wharf, were most at risk.

The Home Office would not comment on the report, but said that security arrangements were constantly reviewed.

"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 office said.

In November, Ms Manningham-Buller said 1600 people were suspected of involvement in terrorist plots against British targets.

Four suicide bombers killed 52 commuters in July 2005 transit attacks in London and security officials say they had foiled at least six other plots. Officials expected the number of plots to increase this year.

The report also said Afghanistan was expected to increasingly become a magnet for Islamic extremists seeking to fight Western military forces.

"With violence in Afghanistan intensifying, and therefore receiving greater media attention, the country may well become more attractive as a venue for foreigners wishing to fulfil their Jihad ambitions," the report said.

The document also revealed that Al Qaeda had grown into a worldwide organisation with a foothold in virtually every Muslim country in North Africa, the Middle East and central Asia.

It said that Al Qaeda's influence extended from North Africa, including Egypt, through to Lebanon, Turkey and Jordan, and into Somalia and Sudan. Al Qaeda was "resilient and effective" in Iraq, its "operating environment and financial position" in Pakistan had improved and a new group had emerged in Yemen.

The paper said the assessment of a resurgent Al Qaeda came two years after Western intelligence said the terror group was virtually a spent force, disrupted by counter-terrorist operations around the world.

In July 2005, the Pentagon obtained a letter written by Ayman al-Zawahiri, Al Qaeda's deputy leader, stating that the organisation had lost many of its leaders and that it had virtually resigned itself to defeat in Afghanistan. Al Qaeda's lines of communication, funding and structure had been "severely damaged".

Jonathan Eyal, the director of international security at the Royal United Services Institute, told the paper the Al Qaeda revival was down to the West's inability to kill or capture Usama bin Laden and that wars in Afghanistan and Iraq made matters worse.

A senior British political source told the paper the picture painted by the document was "particularly bleak and unlikely to improve for several years".