Johnson claims the threat from ISIS and other Islamic terrorist groups is greater than previously reported.
UK Telegraph Until now, it was thought that the main danger came from around 500 jihadis who have travelled to Syria and Iraq from the UK to join Isil or al-Qaeda fighters, around half of whom have returned to Britain.
But the Mayor of London suggested the threat from home-grown terrorist plots was far more widespread than the relatively small numbers of extremists who have gone abroad to fight.
Speaking to The Telegraph, he said: “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.”
Speaking about the Islamic State of Iraq and the Levant (Isil) he 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.”
Mr Johnson gave the interview to promote his new book about Sir Winston Churchill, The Churchill Factor, and suggested the wartime leader would have approved of Parliament’s decision to back air strikes on Iraq.
He said: “Let’s ask ourselves this question: would Churchill now be authorizing air strikes on ISIS? I think he would be. He was a great believer in air power.
“I think he would be appalled by the spectacle of these cowards executing journalists, and I think he would have wanted to do something to set them back a long way, and if need be, to neutralize them altogether. He was a great believer in air power – don’t forget he invented the RAF…so I think he’d do exactly what the Prime Minister is doing.”
He suggests Churchill would have been “very tough” on home-grown terrorism, and would have taken “personal charge” of the situation.
Citing the example of the Sidney Street siege in 1911, when police became involved in a gunfight with a group of anarchists, he said: “If you look at his handling of the Sidney Street siege…Churchill goes down there personally.
“He would have taken personal charge of counter-terror in London, no question. Had he been Home Secretary now, he would be absolutely appalled to think that these people could be carrying British passports and yet be loyal to another state.”