The UK's terror threat level has been raised from severe to critical - the highest level.
The British security services' decision follows a meeting of the high level COBRA group to discuss last night's bombing in Manchester.
It means another attack may be imminent.
Last night's attack at a concert by US pop star Ariana Grande left 22 people dead - including an eight-year-old girl - and dozens injured.
British military personnel could now be deployed to support armed police officers.
British Prime Minister Theresa May said tonight that a wider group may have been involved in the Manchester Arena atrocity - and not just the bomber Salman Abedi:
