Bernard Hogan-Howe, who is retiring as Metropolitan Police commissioner at the end of this month, warned that more efforts were needed to counter the terror ideology.
"The hardest part for the western world is to interrupt this philosophy that Daesh [Islamic State] is perpetuating, which is that Islam in any way supports this horrific use of violence. There is no interpretation I would argue, that could say that, but some people are getting away with that," he told the 'Evening Standard' newspaper.
Around 850 Britons are believed to have travelled to Syria and Iraq to fight for ISIS, with at least half of them believed to have returned to the UK.
The 59-year-old, who is Britain's senior-most police officer, warned that ISIS continued to lure British recruits by using false narratives and these returning recruits posed a grave security threat.
He, however, predicted that Brexit would have a "neutral" effect on Britain's security and crime fighting because it was in the "common interest" of other European countries to cooperate closely with British law enforcement and intelligence agencies.
Disclaimer: No Business Standard Journalist was involved in creation of this content