London Mayor Boris Johnson says the city's security services are monitoring "thousands" of suspected terrorists, a far higher number than had previously been disclosed.
"In London, we're very very vigilant and very very concerned," Johnson told The Telegraph. "Every day — as you saw recently, we had to raise the threat level — every day the security services are involved in thousands of operations."
Previous reports had suggested that around 500 individuals had left the United Kingdom to join Islamic State militants in Iraq and Syria. Of that number, about half were assumed to have returned, with intentions of carrying out terrorist plots in Britain. But Johnson suggested that number might be much higher, and that a large portion of the returnees are setting their sights on London as a destination for carrying out terrorist plots.
"London has a particular concern," he said. "If and when (ISIS militants) come back, we have a real job to deal with them."