BRITAIN is braced for further terrorist attacks following the Manchester atrocity as Theresa May said the threat level was being raised to its highest possible rating.

The Prime Minister has announced a move to deploy the military to protect key sites after investigators were unable to rule out whether Manchester bombing suspect Salman Abedi acted alone.

A Durham Police spokesman said: "Everyone should remain vigilant and report any suspicious activity to the anti-terrorist hotline 0800-789-321, police on 101, or 999 in an emergency."

Mrs May said the government had launched "Operation Temperer", an emergency plan to protect the public which may see military personnel at numerous events in the coming weeks, such as concerts and sports stadiums, working under the command of police officers.

In a live televised statement from Downing Street, the Prime Minister said the terror threat level would be increased to "critical", meaning an attack was expected imminently as it was possible a "wider group of individuals" could have been involved in the Manchester Arena attack rather than just suicide bomber Salman Abedi.

It is understood the 'critical' threat level has been reached twice before - in 2006 to stop a plot to blow up transatlantic airliners with liquid bombs and in 2007 after an attempted London nightclub bombing.

Monday night's attack at a concert by US pop star Ariana Grande left 22 people dead, including an eight-year-old girl, and dozens injured.

Speaking at Number 10 after a meeting of the Government's emergency Cobra committee, Mrs May said: "The spirit of Manchester and the spirit of Britain is far mightier than the sick plots of depraved terrorists.

"That is why the terrorists will never win and we will prevail.

"We again discussed the callous and cowardly terrorist attack in Manchester last night and the operational response from the security service, the police and other emergency services.

"It remains the case that other than the terrorist himself, 22 people were killed in the attack, 59 people remain injured and many of them have life-threatening conditions.

"As Greater Manchester Police confirmed earlier today, the perpetrator was Salman Ramadan Abedi, who was born and brought up in Britain and as the emergency services confirmed throughout the day, his victims were innocent children, young people and their families - our thoughts and prayers are with them all.

"I want to re-iterate what I said this morning about the professionalism of the emergency services and the bravery of the people in Manchester.

"Through their actions, they proved that cowardice will always be defeated by bravery, that evil can be overcome by good and that our values, the liberal, pluralistic values of Britain, will always prevail over the hateful ideology of the terrorists."

Mrs May continued: "In my statement earlier today, I said that the police and security services needed to investigate whether Abedi was acting alone. Those investigations continue.

"But the work undertaken throughout the day has revealed it is a possibility we cannot ignore that there is a wider group of individuals linked to this attack.

"This morning I said that the Joint Terrorism 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.

"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."

Mrs May will chair a meeting of the Government's emergency Cobra committee at 9.30am on Wednesday, Downing Street has announced.