British counter-terror officers are aware of almost 20,000 foreign former jihadi suspects who are still abroad and might enter Britain to carry out attacks.
They are extremists on MI5 files even though they have never travelled to the UK, but can be checked if they do.
All of them are “closed subjects of interest” but, as a recent government paper admitted, any of them could return to being an active threat to Britain “at any time”.
It adds: “This does not mean these SOIs will never pose a threat again, but merely that their current level of threat is not judged to be sufficient to prioritise allocating investigative resources against them.”
Information on threats is fed to MI5 by foreign intelligence agencies, who pass on crucial information to the UK as part of a vast intelligence-sharing network.
Chillingly, there are more than 3,000 “subjects of interest” – people deemed a current threat.
There are several hundred “frustrated travellers” who have been unable to head to Syria or elsewhere to fight jihad.
According to Europol figures, as many as 400 UK jihadis have returned to the country.
Among these returnees just 40 are thought to have been prosecuted.
Round-the-clock surveillance operations are mounted only if suspects appear to be plotting an attack.
Costing £25,000 a day or more, there may be fewer than 20 ongoing.
A Whitehall source said: “Someone who has shown an interest in launching an attack may be diverted into doing something else… and may be temporarily downgraded.
"There are limited resources so teams are reprioritised regularly and tough choices have to be made at the time.”
