News, views and top stories in your inbox. Don't miss our must-read newsletter Sign up Thank you for subscribing We have more newsletters Show me See our privacy notice Invalid Email

The UK terror attack threat level has been increased from severe to critical in the wake of Manchester Arena attack investigation, PM Theresa May has announced.

It means that an attack is expected "imminently".

The last time that the UK threat level was critical was June 30 2007 - in the wake of the Glasgow airport car bombing.

The only time before that it was raised to critical - the highest possible level - was on June 30 2006 when a plot to kill thousands on 10 transatlantic flights from the UK was uncovered.

Anti-terror agencies have been working for the past 24 hours to work out if the suicide bomber who killed 22 and injured 120 more at the Ariana Grande pop concert last night was working alone - or as part of an organised terror cell.

The bomber was earlier named as Salman Abedi.

In a statement delivered at 9.45pm tonight the Prime Minister said that "it is a possibility we cannot ignore that there is a wider group involved".

She also revealed "military personnel could be deployed to support armed police officers" at public events.

Video Loading Video Unavailable Click to play Tap to play The video will start in 8 Cancel Play now

Mrs May revealed that the Secretary of Defence has approved a police request for help from armed forces.

Security Service Mi5 updated its threat level information on its website immediately tonight.

In a live televised statement from Downing Street, she said:"In my statement earlier today I said that the police and security services needed to investigate whether or not Abedi was acting alone."

Video Loading Video Unavailable Click to play Tap to play The video will start in 8 Cancel Play now

"Their investigations continue but the work undertaken during the day have revealed that it is a possibility we cannot ignore that there is a wider group of individuals linked to to this attack.

"This morning I said that the joint terrorist analysis centre - the independent organisation responsible for setting the threat level on the basis of the intelligence available - was keeping the threat level under constant review.

(Image: Mark Worswick / SWNS.com)

“It has now concluded on the basis of today’s investigations that the threat level should be increased - for the time being - from severe to critical

“This means that .. an attack not only remains highly likely but that a further attack may be imminent.

"The change in the threat level means that there will be additional resources and support made available to the police as they work to keep us all safe.

"As a result of JTAC’s decision the police have asked for authorisation from the Secretary of State for Defence to deploy a number of armed military personnel in support of their armed officers.

"This request is part of a well established plan known as operation Temperer, in which both the armed forces and the police officers involved are well trained and well prepared to work in this kind of environment.

(Image: WENN.com)

"The Secretary of State for Defence has approved this request, and operation Temperer is now in force.

"This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces which will allow the police to significantly increase the number of armed officers on patrol in key locations.

"You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police keep the public safe.

"In all circumstances, members of the armed forces who are deployed in this way will be under the command of police officers.

"In the coming days and weeks there will of course be many events hosted up and down the country.

(Image: PA)

"The police will work with the organisers and hosts of these events to come to a judgement about how they can go ahead while making sure the people who attend them are safe and secure. I do not want the public to feel unduly alarmed.

"We have faced a serious terrorist threat in our country for many years and the operational response I have just outlined is a proportionate and sensible response to the threat that our security experts judge we face.

"I ask everybody to be vigilant and to cooperate with and support the police as they go about their important work."

(Image: London News Pictures Ltd)

With the FA Cup final due to be played this weekend, Mrs May acted on the advice of the Joint Terrorism Analysis Centre after chairing two meetings of the Cobra emergency committee over the course of the day.

Mark Rowley, Head of National Counter Terrorism Policing, said the investigation into Salman Abedi is "making good progress".

He added: "However, at this stage it is still not possible to be certain if there was a wider group involved in the attack; 24 hours in we have a number of investigative leads that we are pursuing to manage the ongoing threat.

"We are flexing our resources to increase police presence at key sites, such as transport and other crowded places and we are reviewing key events over the coming weeks.

"As the Prime Minister has already said this evening I have asked for support from the military to be deployed alongside the police.

"This will free up armed officers from certain guarding duties to release our officers to support the wider the response.

"This is part of an agreed and well -rehearsed plan and military personnel will remain under the command and control of the police service.

"At this time we are asking the public to remain calm but alert.

"If you see anything that causes you concern, then let us know immediately, if you have suspicions about someone's behaviour call us.

"Look out for anything that seems out of place, unusual or doesn't seem to fit in with day-to-day life.

"It may be nothing but if you see or hear anything that could be terrorist related trust your instinct no matter how small then ACT and call 999."

poll loading Do you agree with the move to put troops on the streets? 0+ VOTES SO FAR Yes No

Abedi is the first terrorist to successfully detonate a suicide bomb in Britain since four al-Qaeda sympathisers killed 52 people in co-ordinated attacks on London in July 2005.