Northern Ireland has seen its terror threat raised from “substantial to severe”.
The decision was taken by MI5.
The escalation showed an attack is “highly likely”.
Chris Heaton-Harris, the Secretary of State for Northern Ireland, said:
“The threat is monitored and is subject to a regular formal review.
“The public should remain vigilant, but not be alarmed.
“The raised follows an increase in levels of activity relating to Northern Ireland Related Terrorism, which has targeted police officers serving their communities and also put at risk the lives of children and other members of the public.
“These attacks have no support, as demonstrated by the reaction to the abhorrent attempted murder of DCI Caldwell.
“I pay tribute to the tremendous efforts of the Police Service of Northern Ireland and security partners, and the determination and resilience of the Northern Ireland people, who are making Northern Ireland a safer place to live and work.
“The political future of Northern Ireland rests with the democratic will of the people and not the violent actions of the few. Together we will ensure there is no return to the violence of the past.”