UK Security officials are tracking up to 50 British jihadists 24 hours a day with “intense scrutiny” amid fears they could be about to unleash a wave of terror in Britain.
The terrifying claims by intelligence sources come as a former British intelligence officer admitted it was “a case of not if but when” ISIS terrorists strike at home.
MI5 is tracking the plots by “hardcore jihadists” but are struggling with limited resources.
The counter-terror officers are working around the clock to prevent an attack on the scale of Paris or Brussels taking place in Britain.
Investigators fear “sleeper cells” could be awaiting orders at a moment’s notice to carry out plots.
These cells are often small, tight-knit groups that can more easily evade MI5’s network of tipsters monitoring mosques and radical meetings.
Militants in these cells are usually simply awaiting a green light to strike and could attack without any warning to security forces.
Security experts fear in the wake of Paris and Brussels atrocities that London could now be next.
A former British intelligence officer said: “Everyone in the counter-terrorism community believes an attack in Britain will take place.”
There are at least 450 extremists “of concern to counter-terror officials” throughout the UK.
Many have returned from the conflict in Syria and others have been radicalised at home.
The intelligence source added: “The pace of investigations to try and stop this happening is extremely dynamic and fast-moving.
“There is no respite and no sign of the threat to Britain going away.
“The shocking thing about what happened in Paris and the ongoing situation in Brussels is their ability.
“They have shown a high degree of organisation – more than was previously suspected – a strong logistical chain, the ability to get the material needed, guns, explosives, planning.”
Since 2014, Britain has been as a “severe” threat of an attack from extremists, meaning a bombing or shooting on our streets is “highly likely”.
This threat level is one stage away from the top alert state of “critical” which means there is credible intelligence that an attack is imminent.