he UK terror threat level has been raised to its highest level of "critical", meaning further attacks may be imminent, Theresa May has said.
The move came after investigators were unable to rule out whether Manchester bombing suspect Salman Abedi acted alone, the prime minister said.
Military personnel will now be deployed to protect key sites.
Twenty-two people were killed and 59 injured when a suicide bomber attacked Manchester Arena on Monday evening.
The prime minister said soldiers would be placed in key public locations to support armed police in protecting the public.
Military personnel may also be seen at other events over the coming weeks, such as concerts, Mrs May said, working under the command of police officers.
BBC security correspondent Frank Gardner says the number of UK troops deployed will be in the hundreds not around the 5,000 figure being reported.
The prime minister said she did not want the public to feel "unduly alarmed" but said it was a "proportionate and sensible response".
The highest threat level, which is decided by the Joint Terrorism Analysis Centre - a group of experts from the police, government departments and agencies - has only been reached twice before.
Met Police Assistant Commissioner Mark Rowley, who is the national counter-terrorism policing lead, said the investigation was "fast-moving and making good progress".
"However, a critical line of inquiry is whether the dead terrorist was acting alone or part of a group," he said.
"We still have critical lines of inquiry they're chasing down which has led to a level of uncertainty."
The first time the threat level was raised to critical was in 2006 during a major operation to stop a plot to blow up transatlantic airliners with liquid bombs.
The following year, security chiefs raised it once more as they hunted for the men who had tried to bomb a London nightclub, before going on to attack Glasgow Airport.
Former Salford University student Salman Abedi - understood to be a 22-year-old born in Manchester to parents of Libyan descent - is thought to have blown himself up in the arena's foyer shortly after 22:30 BST on Monday.
Fans were beginning to leave a concert by US singer Ariana Grande.
Three of his victims have been named - Saffie Rose Roussos, eight, Georgina Callander - thought to be 18 - and John Atkinson, 28.
So-called Islamic State has said - via IS channels on the messaging app Telegram - it was behind the Manchester attack, but this has not been verified.
