The UK terror threat level has been raised from severe to critical – the highest possible – after the Manchester Arena bombing.
The move indicates that another attack could be “imminent”, as well as “highly likely”.
After chairing a meeting of the Government’s emergency COBRA committee, Theresa May announced the change, saying military personnel will help armed police officers guard key sites.
Under Operation Temperer, members of the armed forces could be deployed to public events, such as concerts and sports events, and also on the streets and at major shopping centres.
Twenty-two people were killed and 59 others injured when a bomber blew himself up in the arena foyer as thousands of concertgoers were leaving after a gig by US pop star Ariana Grande.
The suspect has been named by police as Salman Abedi, and investigations have revealed he may not have acted alone.
Islamic State has claimed it carried out the attack, which involved a homemade device packed with nuts and bolts.
The Prime Minister said the possibility “that there is a wider group of individuals linked to this attack” could not be ignored.
Mrs May said the Joint Terrorism Analysis Centre – the independent body which sets the threat level – had recommended it be raised after the bombing on Monday evening.
She said: “It is 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.”
Assistant Commissioner Mark Rowley, head of national counter-terrorism policing, said the investigation was “making good progress”.
He said: “There has been an arrest and there are currently multiple searches and other activity taking place as I speak.
“The public would expect the police to do everything possible to prevent further attacks and keep them safe.
“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.”