Security services have raised the UK threat level to Critical meaning an attack is "expected imminently". This is an action taken by the Joint Terrorism Analysis Centre (JTAC) not by politicians.
It may suggest that they have intelligence that last night's suicide bomber had accomplices who are at large.
Edited to add - This means the military will be on the streets to assist the police. They will wok under the command of civilian police. Operation Temperer has been invoked...