The threat level from Northern Ireland-related terrorism in Great Britain has gone up from moderate to substantial.
It means an attack in England, Scotland or Wales is "a strong possibility".
Home Secretary Theresa May said the level, set by security service MI5, "reflects the continuing threat from dissident republican activity".
The level for Northern Ireland-related terrorism in Northern Ireland remains severe, meaning an attack is "highly likely".
Despite the increase in the threat level from Northern Ireland-related terrorism in Great Britain, it remains lower than the threat to the entire UK from international terrorism.
This is set at severe - the second-highest of the five ratings used.
But he said the "far bigger" threat to Great Britain came from the ISIS group and its supporters.
Mrs May confirmed the change of threat level relating to Great Britain - meaning three of the four countries of the UK - in a written statement to Parliament.
She gave no further details of any intelligence that had led to the level being changed.
"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," she wrote.
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