|May said 'the first and most important duty of government is the protection and security of the British people' [EPA]
British security services have raised the threat level from Northern Ireland-related violence to suggest an attack is now a "strong possibility" in the mainland United Kingdom.
The threat to Britain is still lower than the overall threat from so-called international terrorism, which remains unchanged at "severe".
"The director-general of the security service has informed me that he has raised the threat to Great Britain from Irish-related terrorism from moderate to substantial, meaning that an attack is a strong possibility," Theresa May, the UK interior minister, said in a statement on Friday.
"Judgments are based on a broad range of factors, including the intent and capabilities of terrorist groups.
"The first and most important duty of government is the protection and security of the British people."
It is the first time the government has published its assessment of the threat posed by Irish-related groups to Britain. A spokesman for the interior ministry would not say if the change was linked to any specific threat.
Jonathan Evans, the head of MI5, Britain's domestic security services, warned last week that dissident republicans opposed to the UK's rule in Northern Ireland could strike mainland UK.
In rare public comments, Evans said there had been a "persistent rise in terrorist activity and ambition" by such groups in Northern Ireland over the past three years.
Northern Ireland has been largely at peace since a 1998 agreement brought an end to three decades of violence between groups seeking to unite British-ruled Northern Ireland with the Irish Republic and Unionists who want the region to remain as part of the UK.
But in recent months, republican dissidents have attacked security forces several times, with the so-called Real IRA believed to be leading much of the campaign including car bombings and shooting at police officers.
Real IRA leaders threatened earlier this month to launch more attacks against "military, political and economic targets".