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11 October 2014, 11:02
Boris Johnson has suggested Britain's security forces are monitoring "thousands" of potential extremists in London alone.
The Mayor of London told the Daily Telegraph people in the capital were concerned about the national terror threat level, raised more than a month ago from substantial to severe, meaning a terrorist attack was ''highly likely''.
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.
"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," Mr Johnson told the paper.
"There are probably in the low thousands of people that we are monitoring in London."
And he added that a large proportion of 500 Britons thought to be fighting alongside Islamic State in Syria were believed to be from the capital.
"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 an when they come back, we have a real job to deal with them," he said.
Meanwhile, in an interview with the same paper, 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 (NSA) 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," he said.