The threat level of Northern Ireland-related terroism in Britain has been raised for the first time in six years.
The risk level has been raised from 'moderate' to 'substantial', meaning an attack in England, Scotland or Wales is "a strong possibility".
Levels are set by security service MI5, and Home Secretary Theresa May said the move "reflects the continuing threat from dissident republican activity".
The level was last raised from 'moderate' to 'substantial' in 2010, before being reduced back to moderate in 2012.
The threat level in Northern Ireland remains at 'severe', which means an attack is "highly likely".
Prison officer Adrian Ismay was murdered in an attack in Belfast in March.
At Easter the New IRA, the group responsible for his death, warned its members were "determined to take the war to the age-old enemy of our nation".
April marked the centenary of the Easter Rising, which was launched by Irish republicans to end British rule in Ireland and establish an independent Irish Republic while the United Kingdom was heavily engaged in World War I.