UK police tracking 25 ISIS terrorist attack plots: Report

British counter-terrorism agencies are tracking as many as 25 serious ISIS terror attack plots threatening to cause carnage in the UK, according to a media report.

More than 50 hardcore jihadists are suspected of planning attacks in Britain in the wake of the Paris and Brussels massacres, the Daily Express reported.

MI5 and British counter-terror police are tracking as many as 25 serious ISIS terror attack plots threatening to cause carnage on Britain's streets, the report said.

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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.

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, the report said.

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.

A former British intelligence officer said, "Everyone in the counter-terrorism community believes an attack in Britain will take place."

There are about 450 extremists "of concern to counter- terror officials" throughout the UK. Some have returned from the conflict in Syria and others have been radicalised at home.

Security experts fear that in the wake of Paris and Brussels atrocities, London could now be next.

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.

"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 intelligence officer was quoted as saying.

Since 2014, MI5 has warned Britain is at "severe" threat of an attack from extremists, meaning that a bombing or shooting on British streets is "highly likely".

This is one stage away from the top alert state of "critical" which means there is credible intelligence that an attack is imminent.