ISIS 'sleeper cells' are reported to be plotting more than 20 deadly terror attacks on Britain's streets in a follow-up to the atrocities in Brussels and Paris.

Intelligence officials at MI5 suspect that more than 50 jihadis - among a reported 450 extremists living in the UK - could be involved in the plans.

Up to 25 serious plots in support of ISIS or Al-Qaeda are said to be being tracked by British security services.

Security fears: ISIS 'sleeper cells' are reported to be plotting more than 20 deadly terror attacks on Britain's streets in a follow-up to the atrocities in Brussels and Paris (file photo)

But there is also reportedly a concern among intelligence chiefs that sleeper cells yet to be detected could launch an attack with no warning, according to The Sun

Around 450 suspects are understood to be the focus of monitoring by the security services, with 50 of those under even more intense scrutiny by MI5 and counter terrorism police.

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

'For some time now it has been a case of not "if" but "when" and 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.'

Last week ISIS threatened to hit the UK 'harder and more bitter' after claiming responsibility for the Brussels bombings.

Extra security: Intelligence officials at MI5 suspect that more than 50 jihadis could be involved in the plans. The Brussels attacks led Britain to ramp up security with armed officers taking to the streets (file photo)

The attacks led Britain to ramp up security with armed officers taking to the streets near major landmarks, train stations and airports.

Yesterday it was revealed that a specially-created 70-strong SAS team will be on standby around the clock ready to fly to the scene of any terrorist incident in this country at a moment’s notice.

The new airborne special forces team, dubbed Blue Thunder, has been rehearsing a number of possible scenarios for months and will be at the command of Home Secretary Theresa May, according to the Sunday Express.

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

Heavily-armed soldiers will be dropped at the site of the attack with a mission to trap and secure any terrorists who remain at large.

Meanwhile more than 2,000 extra armed police officers will be stationed on Britain's streets after an ISIS agent linked to the Paris and Brussels atrocities was found to have taken photographs of a new shopping complex in Birmingham.

The heightened police presence in Britain came after at least 31 people were killed and 270 injured in a series of bomb attacks inside an airport terminal and Metro station in the Belgian capital.

The explosions happened a day after the Belgium Interior Minister warned of possible revenge attacks after the arrest of Paris massacre suspect Salah Abdeslam in the city earlier this month.

Last November 130 people were killed in one night in co-ordinated terror attacks across Paris.