Five Britons are joining Islamic State each week

Metropolitan Police Commissioner Sir Bernard Hogan-Howe cited the figure as he said the militants’ activities in the countries were “not just the horrors of distant lands” and warned of the terrorist threat posed at home by returning fighters.

IS has terrorised parts of the Middle East and shocked the world by posting videos of hostages – including two Britons – being murdered. Around 500 would-be British jihadists are already estimated to have joined the group and Sir Bernard’s disclosure suggests scores more may follow.

Sir Bernard, born in Sheffield and who began his career with South Yorkshire Police, said: “The advance of IS across Iraq and Syria, which happened incredibly quickly, indeed now towards Turkey, are not just the horrors of distant lands.

“We know that over 500 British nationals travelled to join the conflict. Many have returned and many will wish to do so in the coming months and perhaps in future years.

“We still have an average of five people joining them a week. Five a week doesn’t sound much but when you realise there are 50 weeks in a year, 250 more would be 50 per cent more than we think have gone already.

“Those numbers are a minimum. Those are the ones that we believe have gone. There may be many more who set out to travel to another country and meandered over to Syria and Iraq in a way that is not always possible to spot when you have failed states and leaky borders.”

He said the return of “potentially militarised individuals” to the streets of the UK “is a risk to our communities”.

Sir Bernard made the comments during a speech at an event called The National Security Summit in London.

His comments came as it was revealed the family of the third man from Portsmouth to have been killed in fighting in Syria are in the dark about how he died, according to a community leader.

Manunur Roshid was one of five friends who travelled to the war-torn country from Portsmouth, Hampshire, in October last year.

The 24-year-old is understood to have joined up with fighters from the Islamic State. Two of his companions, Ifthekar Jaman, 23, and Muhammad Humidur Rahman, 25, have already died.

Meanwhile, Channel 4 is developing a new drama about British Muslims signing up to fight for Islamic State.

The drama is being made by Peter Kosminsky. It will be made next year.