A leaked top-secret memo from the Joint Intelligence Committee (JIC) says the war in Iraq has “exacerbated” the threat by radicalising British Muslims and attracting new recruits to anti-western terror attacks.

The four-page memo, entitled International Terrorism: Impact of Iraq, contradicts Blair’s public assurances by concluding that the invasion of Iraq has fomented a jihad or holy war against Britain.

It states: “It has reinforced the determination of terrorists who were already committed to attacking the West and motivated others who were not.”

It adds: “Iraq is likely to be an important motivating factor for some time to come in the radicalisation of British Muslims and for those extremists who view attacks against the UK as legitimate.”

The memo was approved by Eliza Manningham-Buller, the head of MI5, John Scarlett, the chief of MI6, and Sir David Pepper, head of GCHQ, the government’s eavesdropping centre.

The leak of the JIC’s official assessment — marked “top secret” — will alarm Blair as it appears to be directed at undermining the public statements in which he has denied that the war in Iraq has increased the terror threat from Al-Qaeda.

In a speech shortly after the London bombings last July, Blair blamed an “evil ideology”, not the war, for motivating the suicide bombers. He said: “If it is Iraq that motivates them, why is the same ideology killing Iraqis by terror in defiance of an elected government?” In a separate speech he dismissed claims that the London attacks were sparked by Iraq, saying: “What they want us to do is to turn round and say, ‘Oh it’s all our fault’.”

He added: “The people who are responsible for terrorist attacks are terrorists.”

At the same time Charles Clarke, the home secretary, accused those who said that the attacks were caused by the war of “serious intellectual flabbiness”.

The JIC report contradicts these ministerial statements. It says: “There is a clear consensus within the UK extremist community that Iraq is a legitimate jihad and should be supported. Iraq has re-energised and refocused a wide range of networks in the UK.”

Written in April last year and circulated to Blair and other senior ministers before the July attacks, it says: “We judge that the conflict in Iraq has exacerbated the threat from international terrorism and will continue to have an impact in the long term. It has reinforced the determination of terrorists who were already committed to attacking the West and motivated others who were not.”

The document says the war is providing an “additional motivation for attacks” against Britain; is “increasing Al-Qaeda’s potential”; and “energising” terrorist networks engaged in holy war. Equally worrying, Iraq is being used as a “training ground and base” for terrorists to return to carry out attacks in Britain and elsewhere.

The JIC is the senior intelligence body in Britain and is responsible for issuing assessments of the gravity of threats to Britain’s national security.

It says that while attacks outside Iraq since the war began in 2003 have not been motivated by the war alone, “in some cases we judge that it has been a major additional motivation”. It cited the example of the 2004 Madrid bombings in which 201 people died, even though, in a speech two months later, Blair denied that those attacks had been sparked by Iraq.

The intelligence committee named Abu Musab al-Zarqawi, the head of Al-Qaeda in Iraq, as a key figure behind the growing threat. It describes him as an “increasingly iconic figure” who is fast becoming the “new [Osama] Bin Laden”. It warns that Zarqawi is seeking to use his status in Iraq to co-ordinate attacks against other countries, including those in Europe.

The JIC analysis presents a disturbing picture of the growth of the terrorist threat and suggests that there is a regular flow of terrorists to and from Britain and Iraq.

“Some jihadists who leave Iraq will play leading roles in recruiting and organising terrorist networks, sharing their skills and possibly conducting attacks. It is inevitable that some will come to the UK,” it says.

A government report, compiled by a senior civil servant using intelligence from the security services and due to be published in the next few weeks, is also expected to recognise that the July 7 bombers were motivated by the invasion of Iraq.

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