At least five Britons are travelling to Iraq and Syria to join Islamic State (IS) every week, the UK's most senior police officer has revealed.
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.
He 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% 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 added: "The drumbeat of terrorism in the UK has changed. It's faster and it's more intense."
Sir Bernard backed plans revealed by the Government last month to give authorities the ability to seize the passports of those they suspect of travelling abroad to fight with terror groups at the border.
He said: "There are, I think, some needs for changes to the law in future. There are some proposals which have been put into the domain which I support.
"The ability to seize passports at the border for a certain period as people leave the UK in part to disrupt their activity and in part to build a case against those who might be getting involved in terrorism abroad."
Possible measures to exclude individuals from returning to Britain are "technically quite difficult" but "definitely something we have got to consider", he said, adding that it is "vital" for airlines to provide information on passenger lists, he added.
He said police activity has escalated to combat the terrorist threat, with "disruption" activities at ports such as stopping suspects and seizing cash up by more than 50%.
"Together with partners we are running an exceptionally high number of counter-terrorist investigations at any one time, the likes of which we have not seen for several years," he said.
The Metropolitan Police have made 218 arrests for terrorist-related activity so far this year. This is an increase of around 70% compared to three years ago, the Commissioner said.
"A large part of this increased arrest rate is due to terrorist activities, plots and planning linked to Syria. The trend is, I think, set to continue,"he said.
Police and partner agencies are disrupting several terrorist plots each year, including at least one "major" conspiracy every year since 2001, Sir Bernard said.
He added that a "heightened threat level to police officers as a target for terror attack" has emerged in recent weeks.
However, he stressed that he believed police have "got the threat under control".
He said: "Yes the drumbeat has changed but we the police and intelligence partners are keeping pace with it, which I hope is a reassurance for the general public."
More than half of terrorism suspects are charged, normally with alleged terrorist offences, he said, adding that courts impose "high and significant" sentences when defendants are convicted.
"We are pursuing with resolution, ruthlessness and vigour the people who would hurt us and it's vital that we maintain that effort," he said.
However, he said Britain "can't arrest our way out of this problem" and highlighted the importance of efforts to prevent radicalisation at source.
A specialist police team is removing more than 1,000 pieces of illegal content from the internet each week, including videos of beheadings, Sir Bernard said. More than 80% of the material taken down is related to Iraq and Syria.
The link between the UK and IS has been laid bare by a string of developments in recent months.
Today it emerged that a man has become the third from the same UK city to have been in fighting in Syria.
Manunur Roshid was one of five friends who travelled to the war-torn country from Portsmouth, Hampshire, in October last year.
Earlier this week the mother of a young British Muslim who went to Syria to fight alongside a group linked to Islamic State (IS) militants has spoken of her "fear and panic" after she travelled to the border of the war zone to retrieve her son.
The woman, known only as Linda to protect her identity, flew to Turkey after her 21-year-old son left their home in north London and entered the war-torn region without her knowledge
There have also been appeals to try to find missing teenagers Samya Dirie, 17, and Yusra Hussien, 15, who vanished last month and are thought to have travelled to Syria.
And a London-accented man known as Jihadi John has appeared in IS videos presiding over the beheading of hostages.
Sir Bernard made the comments during a speech at an event called The National Security Summit in London.