The Met Commissioner has warned around 250 British jihadis who have fought in Syria and Iraq are now back in Britain and have joined a "network of terrorists".
Sir Bernard Hogan-Howe said majority of the men believed to be from London posed a serious risk to home security.
"The thing we're worried about is so many people from Britain have gone to Syria and potentially Iraq to get involved in terrorism. That's a criminal offence. We think at least 500 to 600 went. About 250 have come back," Hogan-Howe told the Mirror.
"They risk being militarised and they are in a network of terrorists. It's a worry because we think two- thirds to three-quarters come from the London area, so that's quite a large number."
The commissioner spoke as Brit Isis (also known as Islamic State) jihadist Amu Baker vowed to unleash terror in the UK.
Speaking on a grainy video the masked rebel said: "If there's no other chance than to come back...then I'll have to do that..I'm ready to take that step to come back if your armies, if your countries don't stop attacking us."
Another British jihadist, called Abu Anwar al-Britani boasted he would take part in sick executions of innocent people like that of US journalist James Foley who was beheaded by a masked figure - dubbed John the beatle - who is a key suspect in an international manhunt.
Al-Britani said of Foley's murder: "My initial reaction personally was that this was a direct justified response to the crimes of the US against the Islamic State.
"I would be more than honoured to partake in an execution like this. I hope God gives me the chance to do such a thing as the brother did with James Foley, a soldier of Bashar or a soldier of America, my hands are ready to do this blessed act."
Hogan-Howe said the Met was working with the US on this murder case and assured progress was being made but that "it was not straightforward" - investigating claims about whether or not the suspect was a British citizen.
The policeman also supported growing calls for Britons fighting in war zones such as Syria and Iraq to be stripped of their citizenship and wants the government to invest more money into preventing terrorism on our streets, such as bringing back the abolished control orders for suspects who cannot be charged or deported. They were replaced with the watered-down Terrorism Prevention and Investigation Measures, also known as Tipims.
"They were stopped because the threat was reduced and quite properly they were seen as too intrusive. But I think these things have got to be considered when the drum beat changes, and it's clear it has," he added.