Police are now arresting a suspected terrorist every day with ministers poised to boost funding amid fears of attacks from Isil “misfits and criminals”.
Metropolitan assistant commissioner Mark Rowley revealed fanatics were being detained on a daily basis following a sharp rise in arrests because of the growing threat from Islamist jihadists.
Mr Rowley, the country’s most senior counter-terrorism officer, said he had been lobbying Government for additional funding to tackle the threat and there were “indications” that would happen.
He said the greatest danger to the UK were “misfits, criminals and vulnerable” who were being targeted by Isil to carry out atrocities on home soil.
Police are also taking down 1,000 web pages of extremist material every week.
More than 600 British jihadists are believed to have gone to Syria or Iraq and half of them have already returned.
Police were not previously aware of 300 of them, Mr Rowley warned.
“We are not just dealing with a classic terrorist organisation, organising plots across the world,” he told the BBC Andrew Marr show.
“We are dealing with a group trying to create a corrupt cult of people, of followers who will act in their name.
“They are trying to attract misfits, criminals and the vulnerable. It is these people, not part of the larger organisation who might act of their own volition.
“That is the challenge on us to have good sight of them and intervene with them.”
The risk posed by the self-starting extremist or terrorist is the gravest concern of the police and security services.
Last month, Andrew Parker, the Director General of MI5, said a terror attack on the UK was almost inevitable because the intelligence agencies and police could not expect to stop every single plot.
And following the terrorist attacks in Sydney and Paris in the last two months, David Cameron, the Prime Minister, warned they could just as easily happen in the UK.
Figures show a total of 327 people were arrested for terrorism offences last year, an increase of a third on the previous year.
They included a sixfold increased in Syria-related arrests from 25 to 165.
Mr Rowley said: “We are disrupting people day in, day out.”
He said he shared Prince Charles’s fears of how young people in the UK were being radicalised in their own communities, warning: “The dynamic that worries us most of all is the ability of Isil to reach in to communities.”
He said more money was needed to grow the anti-terrorism unit over the next year and that discussions with the Government for funding were ongoing.
Asked if that meant he expected additional resources, he said: “That is the indication at the moment.”
The senior officer confirmed emergency plans to cope with a terror attack in the UK had been changed in the wake of the Paris shootings.
“In terms of our national firearms capability, we’ve asked is it strong enough? How’s it placed? How’s it organised?” he said.
“We’ve arranged to be able to deal with those sorts of events and we have some well-tested exercises and command and control regimes for working across the country on counter-terrorism.
“But you look at an event like Paris and you think not everything in that we anticipated, so we’re going to have to make some refinements to our plans to improve.”