AN ATTACK in Britain by Northern Irish terrorists is now a "strong possibility" after the Home Secretary raised the terror threat level to 'substantial'.
The surprise move raises the threat level from 'moderate', meaning England, Scotland and Wales are considered to be at an increased risk of an IRA-style bombing.
Mrs May said the level "reflects the continuing threat from dissident republican activity".
MI5 takes responsibility for setting the country’s terror threat level, which remains at ‘severe’ for the UK as a whole from international terrorism.
Mrs May gave no indication of a specific plot that could have forced MI5 to reevaluate the threat level from Northern Ireland.
In a statement to the House of Commons this afternoon, she 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."
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."
The Home Secretary raised the threat level for Northern Ireland terrorism from 'moderate' to 'substantial' in 2010 but it was dropped back to 'moderate' in 2012.
The most recent dissident republican attack was in March, when prison officer Adrian Ismay was murdered by a group calling itself the new IRA.