Theresa May has announced the terror threat level in the UK is being raised to "critical" and soldiers will now be deployed on the streets to protect key sites, in a significant escalation of the policing response following the Manchester attack.
It is the first time in 10 years the feared threat of a terror attack has reached its highest level and means some 5,000 troops could be deployed to support police, including at concerts and sporting events.
The Prime Minister said police in Manchester were working to establish whether arena bomber Salman Abedi, 22, was working alone when he killed 22 people and injured 59 others in a suicide explosion on Monday night.
However, she added: "The work undertaken throughout the day has revealed that it is a possibility that we cannot ignore, that there is a wider group of individuals linked to this attack."
The threat level has been raised "for the time being", meaning "a further attack may be imminent", she said.
In a live address late on Tuesday, Ms May said: "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 [the Joint Terrorism Analysis Centre'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.
"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 to 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."
The UK's terror threat level has been set at "severe" for some time.
The last time it reached "critical" was in 2007.
Ms May added: "While we mourn the victims of last night’s appalling attack, we stand defiant. 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."
Assistant commissioner Mark Rowley, of the Metropolitan Police, said counter-terror police were making "really good progress" in a "massive investigation" into the Manchester attack.
Mr Rowley, the country's most senior anti-terror cop, said: "We still have some critical lines of enquiry that we're urgently chasing down. But that leaves a degree of uncertainty which has led [JTAC] to independently judge that the threat level needs to be raised to critical, meaning a further attack may be imminent.
"The public would expect the police at this moment to be doing extraordinary things, everything possible to protect them. The first thing we do is stretch our police resources. We have mobilisation plans, we cancel leave, we stretch shifts. We can double the number of officers on the streets, both armed and unarmed.
"But on top of that, over the last couple of years looking at experiences elsewhere in Europe, we've set up a contingency plan with the military where the army will step forward.
"All those extra police officers that we're putting on the streets already will be augmented by military support. The first phase, those military officers will take over some of our guarding duties at key fixed locations.
"And potentially subsequent to that we could even use military support police at key locations or perhaps at key events."
Mr Rowley would not be drawn on whether he personally believed another attack was imminent, but spoke of "uncertainty currently flowing from the investigation". JTAC, he said, had decided that "whilst we chase that down, it makes sense to raise the threat level".
According to a review of London's terror preparedness from 2016, "only in the most extreme situations would the military be deployed in routine patrolling of the streets of London" under Operation Temperer.
Lord Harris' review added: "In addition, specialist troops can be authorised to take part in a direct operation to confront and neutralise a terrorist threat if required. This increases the police capacity and capability to respond, for example, if there were multiple attacks on different sites of the kind seen in Paris in November 2015."
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