Britain's security level has been raised to its highest possible level as the country braces itself for further terrorist attacks after last night's Manchester suicide bombing.
In a live televised statement from Downing Street, Theresa May said the terror threat level would be increased to "critical", meaning an attack was expected imminently.
The Prime Minister said it was possible a 'wider group of individuals' could have been involved in the Manchester Arena attack rather than just suicide bomber Salman Abedi.
In a sign of the increased threat, the military could be deployed to support armed police officers, Mrs May said.
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, 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."
Mrs May will chair a meeting of the Government's emergency Cobra committee at 9.30am on Wednesday, Downing Street has announced.
Thousands of people gathered in Manchester's Albert Square this evening to remember the victims, in scenes repeated across the country.
The terror threat level was increased after investigations revealed Abedi may not have acted alone.
Mrs May said: "The work undertaken throughout the day has revealed that 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.
"This means that their assessment is not only that an attack remains highly likely, but that a further attack may be imminent."
The Prime Minister said Operation Temperer - allowing military personnel to take to the streets - was 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."