Intelligence services have identified 350 potential terrorists who have returned to Britain from fighting in Syria, sources have told the Times.
The figure was disclosed to the newspaper to demonstrate the strain being placed on counter-terrorism agencies that place possible threats under round-the-clock surveillance to avoid an attack on British soil.
Keeping just one person under watch requires between 24 and 30 intelligence officers, according to the source.
Some returning extremists need additional surveillance as they have picked up training to avoid being electronically tracked, meaning they require physical monitoring too.
Andy Hayman, who was head of specialist operations at Scotland Yard at the time of the London 7/7 attacks, told the Times that the increase in the number of potential suspects to be monitored was "shocking".
"I recall the number of people we had to keep under 24/7 watch in the wake of the 2005 attacks – it was far, far smaller than this, yet the drain on our resources was huge," he said.
The level of appropriate surveillance for each potential extremist is established on a case-by-case basis with some returning jihadists quickly becoming disillusioned with radical Islam, while others plan serious plots.
Despite the wide array of tools available to intelligence and counter-terrorism services, they say it can be difficult to bring suspected terrorists to court. Evidence is often gathered by covert means, making it inadmissible in court. Intelligence officers are also wary of identifying their sources and informants as this would mean they cannot be used again.
As a result, some extremists remain free to roam the streets of Britain and preach their message of hate to vulnerable and malleable youths.
MI5, Britain's interior intelligence network, believes that there are more than 3,000 Islamist extremists in Britain, according to the Times.
The warning comes following recent activity including the Westminster attack, in which Khalid Masood killed five people; the arrest of a man armed with knives in Whitehall; and an anti-terror raid in north-west London.