Security services have identified 350 Islamic State militants who have returned to Britain from Syria and who pose a potential terror threat.

This is in addition to the 3,000 violent Islamist extremists that MI5 has confirmed are already in the country, Whitehall sources have told The Times.

MI5 has said 850 people have traveled from the UK to Islamic State-controlled parts of Syria. Whilst some that have returned are designated as ‘disillusioned with extremism’ and allegedly pose no danger, a large number subscribe to Islamist ideology and are trained to handle firearms and explosives.

At the end of 2016, the Belgian interior minister said a “wave” of between 3,000 and 5,000 Islamic State jihadists who have European passports could return to the continent, as the terror group begins to lose territory in Iraq and Syria.

The newspaper reports that keeping one jihadist under 24-hour surveillance requires between 24 and 30 police or intelligence officers, and puts enormous strain on intelligence services.

High-risk suspects who are believed to be engaged in planning attacks need to be subject to man-hour intensive physical surveillance as they are most aware of techniques to avoid electronic eavesdropping by GCHQ and MI5.

In late 2015, following the Charlie Hebdo and Bataclan attacks, it was revealed by then Prime Minister Manuel Valls that French security services had 20,000 people on the terror watch list in France. More than half of those with a surveillance ‘Record S’ belong to or were linked to an Islamist movement – a number of extremists essentially impossible to monitor comprehensively.

The Whitehall source said that many returning Islamists cannot be prosecuted because the information identifying them has been obtained through covert means, meaning it may be inadmissible in court or could endanger sources if evidence is disclosed.

The source also noted the added the risk of returning jihadists radicalising young Muslims in their communities, indoctrinating them to commit acts of terror on British soil or to become foreign fighters in Syria.

Andy Hayman, who was head of Specialist Operations at Scotland Yard when the 7/7 London attacks occurred and when a series of Islamist terror plots were thwarted, said that the increase in the number of potential suspects was “shocking”.

He added: “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.”

Security services have been on high alert following the Westminster terror attack in March. In late April, a suspect armed with knives in Whitehall was apprehended and an active terror plot was foiled with the suspected terrorist cell raided in north-west London in anti-terror operations.