MI5 has raised the official threat level to Britain from Northern-Ireland related terrorism, Home Secretary Theresa May has announced.
The Security Service increased the level from moderate to substantial - which is the third most serious category out of five and means a terrorist attack is seen as a "strong possibility".
Mrs May said the move "reflects the continuing threat from Dissident Republican activity".
In a statement to the House of Commons, she said: "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 threat level to the UK from international terrorism remains at severe - meaning an attack is "highly likely". This has not been changed.
Mrs May said the threat level to Northern Ireland from Northern Ireland-related terrorism is also unchanged at severe.
She added: "The public should remain vigilant and report any suspicious activity to the police."