Isolated communities, segregation and unregulated schooling in the UK are a “breeding ground” for extremism and future jihadists, a counter-terrorism police chief has warned.

Metropolitan Police Deputy Assistant Commissioner Neil Basu said there was a “definite problem” of second-generation Britons who became radicalised through the “very toxic combination” of isolation and extremist online content.

Police are dealing with nearly 600 active investigations after a “summer like no other” that saw major attacks in Manchester and London that left dozens of people dead and more than 200 injured, he said.

The attacks led to a “massive spike” in the number of calls to the national terrorism hotline, trebling the number of leads.

Meanwhile the number of new investigations launched since March has surged by 75% to 65, while police have foiled five attacks.

Security services currently have 3,000 open “subjects of interest”, while there are 20,000 previous subjects of interest, Mr Basu said, warning: “Those numbers are just going to keep increasing.”

Addressing the Police Superintendents' Association conference, he said fears that the fall of Isis in the Middle East would lead to a rise in attacks from overseas had been replaced by the danger posed by home-grown jihadists.

“The threat was the traveller or the returning fighter, who was battle-hardened and even angrier, but now it's the threat in our midst,” he said.

“We stopped a lot of those would-be jihadists travelling too and some of those remain committed to their cause. If they can't travel, then why not attack us here.

“There is also a definite problem in segregated and isolated communities and with what I think is an even more extreme second generation.”

Mr Basu said disenfranchised groups were being radicalised by propaganda delivered in “six-second soundbites through their handheld devices 24/7”.

“Segregated and isolated communities, unregulated and home schooling are a breeding ground for extremism and future terrorism,” the officer said.

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As a result police find it very difficult to profile and identify any would-be attackers, as they come from a variety of backgrounds and are of all ages and genders.

“They have been the educated, they have been the illiterate and they have been the completely unknown,” Mr Basu said.