Britain's counter-terrorism chief has warned officers are grappling with exceptionally high numbers of investigations as they foil several murderous plots a year.
With fears growing over the influence of extremist group Islamic State (IS), Mark Rowley, who is the UK's National Policing Lead for Counter Terrorism, revealed details of the scale of the deadly threat.
It came as the UK Government announced it is sending pilotless drones to Iraq to help fight IS.
Mr Rowley said hundreds of suspected terrorists have been arrested so far this year, and police are removing more than 1,000 pieces of extremist material from the internet each week, including videos of beheadings, torture and suicides.
Scotland Yard Assistant Commissioner Rowley said: "This year we have made 218 arrests and are running exceptionally high numbers of counter-terrorism investigations, the likes of which we have not seen for several years… We are disrupting several attack plots a year.
"These plots are of varied sophistication, from individuals planning to carry out spontaneous yet deadly attacks to more complex conspiracies. Almost all seemingly are either directed by or inspired by terrorism overseas. We have secured evidence and charged 16 returnees for terrorist activity in Syria.
"The volume, range and pace of counter-terrorism activity has undergone a step-change."
More than 80 per cent of material removed from the internet is related to Iraq and Syria, and Mr Rowley said so far 66 people who have been reported missing to police were feared to have travelled to Syria. They included Aqsa Mahmood, 20, from Glasgow, who has not been seen for months since she apparently fled to Syria to marry an IS fighter.
The Government said the use of Reaper drones will complement the RAF Tornado strike force. Defence Secretary Michael Fallon told MPs: "As the UK's only armed remotely piloted aircraft, Reaper will add to the strike capability we are already providing."
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