MI5 has raised the threat level to the UK from Northern Ireland-related terrorism from "moderate" to "substantial".
It is the third most serious category out of five and means a terrorist attack is thought to be a "strong possibility".
Home Secretary Theresa May told the House of Commons the change "reflects the continuing threat from dissident republican activity".
She added: "As a result of this change, we are working closely with the police and other relevant authorities to ensure appropriate security measures are in place."
In a statement, the Home Office said: "The Security Service, MI5, has increased the threat level to Great Britain from Northern Ireland-related terrorism from moderate to substantial.
"This means that a terrorist attack is a strong possibility and reflects the continuing threat from dissident republican activity.
"As a result of this change, we are working closely with the police and other relevant authorities to ensure appropriate security measures are in place."
The Home Office advises members of the public to remain vigilant and report any suspicious activity.
The threat level to Northern Ireland from Northern Ireland-related terrorism has not changed from "severe".
UK threat level from international terrorism remains at "severe", which means an attack is "highly likely".
