UK terror threat level raised to critical, meaning an attack is imminent, Prime Minister says

The UK terror threat level has been raised from 
severe to critical, the highest possible level, meaning an attack 
is “expected imminently”.

The Prime Minister announced the threat level had been raised after the Parsons Green tube bomb attack

It means that troops will patrol the streets and security will also be stepped up at major public events and terrorist targets such as railway stations and airports.

Troops will bolster police forces as police hunt for the Parsons Green bomber.

Isil have claimed responsibility for the London attack.

Prime Minister Theresa May  said military personnel would replace police officers "on guard duties at certain protected sites which are not accessible to the public".

She said: "The public will see more armed police on the transport network and on our streets, providing extra protection.

"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."

The last time the threat level was raised to critical was in May, in the direct aftermath of the Manchester bombing, but it was reduced again three days later. That was the first time in ten years, the UK was on its highest alert.A manhunt is under way after the improvised explosive device (IED) sent a fireball through a packed London Underground train carriage in south west London, during the Friday morning rush-hour.

The Joint Terrorism Analysis Centre, which was established in 2003 and is based at MI5's London headquarters, decides the threat level.

A number of factors may be taken into account, including available intelligence, terrorist capability, terrorist intentions and timescale.

Rita Katz, the director of US-based Site Intelligence, said IS claimed the bombing was the work of a "detachment" rather than simply a "soldier", which she said implied it was a co-ordinated attack.

The terror group has frequently declared that it was behind terrorist incidents in recent years but experts have warned their claims should be treated with caution.

It was the fifth terror attack on British soil in six months.

Images taken by commuters caught up in the atrocity appeared to show that fairy lights had been used as a detonator in the bomb made inside a bucket and carried inside a plastic Lidl carrier bag.

It is believed the device had a timer but detonated unexpectedly as the bomber fled the carriage.

A total of 29 patients needed hospital treatment, many of whom suffered burns. Others were injured in the panicked stampede to flee the station.