As many as one in 11 Muslims in Britain agree with and proactively support terrorism
Haras Rafiq also told officers at Scotland Yard that up to 20 per cent of the Muslim population ' sympathise' with militants, while stopping short of being prepared to 'blow themselves up'.
His remarks underline the scale of the task facing Gordon Brown to win the hearts and minds of Muslims, only a week after he promised an extra £70million to councils and community groups to fight extremism.
Mr Rafiq, an adviser to the Government's preventing extremism taskforce, said: "A percentage of people actually agree and support proactively the people that are deciding to blow themselves up.
"It varies, it can be 7 per cent, 5 per cent, 9 per cent."
With 1.6million Muslims living in the UK, nine per cent is the equivalent of 144,000 people supporting terrorism.
'Proactively' supporting terrorism is understood to mean the people are vocal in their support for fanatics, rather than actively helping them to commit atrocities.
Mr Rafiq, filmed by Channel 4's Dispatches, went on: "Next we can have a percentage of people that can actually sympathise.
"These people at this stage ... won't go out and be operational and won't decide to blow themselves up but can sympathise with the people that can blow themselves up.
"Again this can be in double digits. Then we have a percentage of the population that actually empathises with the people that blow themselves up. It could be 15 to 20 per cent.
"It could be somebody who says, 'I don't agree that these guys are blowing themselves up but I can actually understand why'."
Mr Rafiq, a member of the Sufi Muslim Council, made his presentation to Scotland Yard earlier this year, after almost two years of Government attempts to combat extremism in the wake of the July 7 bombings.
Critics argue that working parties and an attempt to persuade Muslim leaders and groups such as the Muslim Council of Britain to speak out against extremism have not had the desired impact.
The documentary reveals that fanatics are continuing to peddle a message of hatred in the UK.
This centres on persuading Muslims that the covenant of security that in return for safety and freedom, Muslims do not attack the nation that is their home - has been broken by draconian anti-terror laws and the war in Iraq.
Dispatches discovered that Abu Mohammed, a fanatical preacher based in Europe, managed to make a series of visits to Britain, lecturing to young Muslims in houses in Luton, before being banned by the Home Office in March this year.
He continues to radicalise followers here via the Internet.
Mohammed is filmed by reporter Phil Rees declaring: "We are in a state of war and no covenant exists...British Muslims should know that the British Government will do everything to frighten them, to make them very uncomfortable and they have to be prepared to pay the price and fight back."
Home Secretary Jacqui Smith said yesterday that the Government is changing its approach to dealing with extremism.
She added: "It seems to me that what we should be doing is emphasising the values that we share which are under attack from terrorism-rather than trying to create a battle or war between those who oppose the terror and those who want to carry it out."
She suggested Muslim communities have not been best served by their leaders.
She backed moves, put in place by Ruth Kelly when she was Communities Secretary, to broaden the kinds of groups with which the Government engages.
Miss Smith told New Statesman magazine: "We've got to make serious attempts to go beyond those who have previously been seen as leaders of the community."
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