British Prime Minister Theresa May says the terror alert level in the UK has been raised from 'severe' to 'critical' in the wake of the Manchester attack.
The raising of the alert indicates that a further attack is 'expected imminently'.
The prime minister warned authorities could not ignore the possibility that a 'wider group of individuals' was linked to the attack in Manchester which killed 22 people.
'I do not want the public to feel unduly alarmed. We have faced a serious terrorist threat in our country for many years,' Ms May said in London.
She said armed police guarding key sites would be replaced by military personnel to allow more police resources to be freed up.
Members of the armed forces will also be deployed alongside police at some events including concerts and sporting events.
'The operational response I have just outlined is a proportionate and sensible response to the threat that our security experts judge we face,' she said.
The independent body which sets the threat level had recommended it be raised after a man named by police as Salman Abedi set off an improvised bomb on Monday night as crowds streamed out of a concert.
The Department of Foreign Affairs and Trade's travel advice for the UK has been updated, with Australians urged to exercise a high degree of caution in the country in the light of the increased threat level.
'Be vigilant with your personal security, monitor the media and follow local authorities' instructions,' the advice says.
