Image copyright Pacemaker 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.
BBC security correspondent Frank Gardner said it had been 15 years since the last Northern Ireland-related attack in Great Britain, but there was now a "real fear" in the security services that deadly tactics of the past would be used again.
But he said the "far bigger" threat to mainland Britain came from the Islamic State group and its supporters.
'Little support'
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 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.
In a separate statement, Mrs May said dissident republicans "command little support", and the "vast majority" of people wanted peace.
She said it was "sensible" for the public in Great Britain to be vigilant, but said the threat level should "not affect how we go about our daily lives".
At Easter the New IRA, the group responsible for the murder of prison officer Adrian Ismay in March, warned that its members were "determined to take the war to the age-old enemy of our nation".
The threat level for Northern Ireland-related terrorism in Great Britain was raised from moderate to substantial in 2010, then reduced back to moderate in 2012.
The top threat level is critical, which means an attack is "expected imminently".