London Mayor Boris Johnson has revealed that thousands of terror suspects are being spied on every day by UK security services.
He said the number of suspects in London alone is said to be in the "low thousands", suggesting the threat from home-grown terrorism may be far more widespread than that from Syria or Iraq.
Mr Johnson told the Daily Telegraph: "In London we're very very vigilant and very very concerned. Every day - as you saw recently, we had to raise the threat level - every day the security services are involved in thousands of operations.
"There are probably in the low thousands of people that we are monitoring in London."
Until now the main threat was thought to come from about 500 suspected jihadists who went to Syria or Iraq from the UK to join Islamic State militants.
Speaking about IS, Mr Johnson said: "London has a particular concern, because probably of the five or six hundred that are out there, we think a third, maybe more - maybe half - come from the London area.
"If and when they come back, we have a real job to deal with them."
It comes as police chiefs also issued a warning to officers to increase vigilance in the face of a heightened terror threat for officers and five men were being held by Scotland Yard suspected of plotting a potentially "significant" attack on the UK.
Meanwhile, in an interview with the same newspaper, Iain Lobban, director of intelligence agency GCHQ, hinted it now took three times longer to trace terrorists online in the wake of disclosures made by former National Security Agency contractor Edward Snowden.
And he said sharing intelligence with a greater number of nations was necessary.
"It's nonsense not to share with the French. This is not Blitz Britain. We sure as hell can't lick terrorism on our own," Mr Lobban said.
