British Home Secretary Theresa May has said the threat level from dissident republicans has gone from moderate to substantial, meaning an attack is a 'strong possibility'.
The Home Office said it is the first time that this threat level has been published. The threat from international terrorism remains severe.
Mrs May said: '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.
'Judgments are based on a broad range of factors, including the intent and capabilities of terrorist groups.
'This is the first time we have published the Irish-related threat assessment to Great Britain. This is in the interests of transparency and to encourage people to remain vigilant.'
There are five threat levels and substantial is the third ranked level. The other levels are low, moderate, critical and severe.
Mrs May added: 'The first and most important duty of government is the protection and security of the British people.
'We have been consistent in stating that the threat to the UK from terrorism is real and serious.'
The head of MI5 warned last week that dissident republicans could attempt to mount a new wave of terrorist attacks on the British mainland.
Jonathan Evans, the director-general of the Security Service, said there had been a 'persistent rise' in 'activity and ambition' by dissident groups in Northern Ireland over the past three years.
While they did not have the capacity to return to the levels of violence during the Troubles, he said they still represented 'a real and rising security challenge'.
For several months both the Gardai and the PSNI have made references to the increased level of threat.
