The man who carried out a suicide attack in Manchester was "likely" to have not acted alone, Home Secretary Amber Rudd says.
Salman
Abedi killed 22 and injured 64 when he blew himself up at the
Manchester Arena on Monday night - 20 people are in critical care.The UK terror threat level is now up to its highest level of "critical", meaning more attacks may be imminent.
It means military personnel will now be deployed to protect key sites.
Four of the victims are known to be eight-year-old Saffie Rose Rousses, Olivia Campbell, 15, John Atkinson, 28, and Georgina Callander - thought to be 18.
According to Reuters, the Polish foreign minister has said that two Polish people who went missing after the attack are among those killed.
Mrs Rudd said the attack was a "devastating occasion".
"It was more sophisticated than some of the attacks we've seen before, and it seems likely - possible - that he wasn't doing this on his own," she added.
- Manchester attack: Latest updates
- Victims of the attack
- What we know so far
- Theresa May's speech in full
- Who was the suspect Salman Abedi?
She also said up to 3,800 troops will be deployed on the streets around the UK.
Mrs Rudd also said there would be an "uplift" in Prevent, the government's anti-radicalisation programme, after June. This had already been planned before Monday's attack, she added.
The change in terror threat comes after investigators were unable to rule out whether the bomber, named by police as Salman Abedi, was part of a group.
He is understood to be a 22-year-old born in Manchester to parents of Libyan descent.
The prime minister said soldiers would be placed in key public locations to support armed police in protecting the public. These include Buckingham Palace, Downing Street, embassies and the Palace of Westminster.
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.
The prime minister said she did not want the public to feel "unduly alarmed" but said it was a "proportionate and sensible response".
A hotline has been set up for people concerned about loved ones - 0800 096 0095.
And anyone with information about the attack can call the anti-terror hotline on 0800 789321.
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.
The Metropolitan Police says it has increased its presence across London, including specialist police officers who are trained "to spot the tell-tale signs that a person may be carrying out hostile reconnaissance or committing other crime... based on extensive research into the psychology of criminals and what undermines their activities".
Fans were beginning to leave a concert by US singer Ariana Grande.
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.
Analysis
By Dominic Casciani, BBC home affairs correspondent
This is a major and very rare move by the UK's security chiefs.For the past few years, ministers, police chiefs and others have been at pains to try to warn the public that the threat faced by the UK was severe.
But they have steered clear of warning, even when a major plot was being tracked, that an attack could be close. This time, they feel they have no choice other than to say it may be imminent.
In short, nobody at this stage can say for sure whether Abedi acted alone or with the help of others. They can't rule out if there are other people out there.
What does it mean for us, the public?
Some of us will see the Army in key locations that need guarding so that armed police can be freed to focus on policing rather than guarding.
We should expect additional time-consuming security checks at ports and so on.
The aim for security chiefs is to ratchet up the security while keeping the country moving.
The wounded, who include 12 children aged under 16, are being treated at eight hospitals across Manchester.
Several people are still missing, including Eilidh MacLeod, 14, from Barra in the Outer Hebrides, Chloe Rutherford, 17, and Liam Curry, 19.
Eilidh's friend, Laura MacIntyre, 15 - who was also reported as missing - was later identified as one of the seriously injured in a Manchester hospital.
Manchester metro mayor Andy Burnham told the BBC that the attack had been the city's "darkest hour".
Home Secretary Amber Rudd, Labour leader Jeremy Corbyn and Commons Speaker John Bercow stood on stage alongside Manchester Mayor Andy Burnham and Greater Manchester Police Chief Constable Ian Hopkins.
Witnesses at the arena described seeing metal nuts and bolts among the debris of Monday's bomb, and spoke about the fear and confusion that gripped concert-goers.