Prime Minister Theresa May announced that the UK's terror threat level has been raised to critical from severe, which means that a terror attack is expected imminently. Armed forces would be deployed across the UK to support armed police officers as part of the decision to raise the terror level, she added.
The move comes a day after a suicide bomber killed 22 people and injured 59 others at the Manchester Arena on 22 May.
"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," May said in a statement after a meeting of Cobra, the government's crisis response committee on Tuesday (23 May) evening.
She said armed forces personnel, who will be under the command of the police, will be visible at "big events" such as football matches and concerts.
Operation Temperer will allow military personal to take to the streets. "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," May said.
In her statement May added that the bravery of the emergency services and the people in Manchester shows that cowardice will "always be defeated by bravery, that evil can be overcome by good and that our values, the liberal, pluralistic values of Britain, will always prevail over the hateful ideology of the terrorists."
While not wishing to unduly alarm the public, May said that the response was proportionate and appropriate. "We stand defiant ... the terrorists never will, and we will prevail," The Guardian reports.
Major and rare move
BBC's home affairs correspondent Dominic Casciani described the move as "a major and very rare move" by the UK's security chiefs.
"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 [Salman Ramadan] Abedi acted alone or with the help of others. They can't rule out whether there are other people out there."
Third time UK at critical terror threat level
The BBC says this is the third time that the UK has been placed at the critical terror threat level - the highest since the alert system was first introduced.
In 2006, the critical level was triggered when a plot to blow up transatlantic airliners with liquid bombs was foiled. The following year, the threat level was upped to critical again after a man tried to bomb a London nightclub, before trying to attack the Glasgow Airport.
More people may be involved in Manchester attack
Theresa May also said that it was possible that there is a "wider group of individuals" involved in the Manchester explosion.
She did not give details but BBC notes that due to the complexity of the bomb produced by Abedi, it is thought that "there would be a network of collaborators.