The UK terror threat level has been raised to its highest level of "critical", meaning further attacks may be imminent, Theresa May has said.
Military personnel will now be deployed to protect key sites as a result of the new threat level.
Meanwhile, the mother of Olivia Campbell said her daughter died in Monday night's Manchester bombing, which killed 22 and injured 59.
Writing on Facebook, she paid tribute to her "precious" daughter, aged 15.
Olivia's mother, Charlotte Campbell, had issued an emotional plea for information on her daughter's whereabouts after the Ariana Grande concert.
And according to Reuters, the Polish foreign minister has said that two Polish people who went missing after the attack are among those killed.
The change in terror threat comes after investigators were unable to rule out whether suspect Salman Abedi acted alone, the prime minister said.
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".
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.(Source: BBC)