An attack on Great Britain by Irish-related terrorists is a "strong possibility", Home Secretary Theresa May said as the country's threat level was raised to substantial.
The threat assessment was increased after the head of the MI5 warned that dissident Irish republicans could attempt to mount a new wave of terrorist attacks on the British mainland.
But it was still lower than the overall threat to the UK from international terrorism, published by the Joint Terrorism Analysis Centre, which remains at severe, meaning an attack is highly likely.
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."
The assessment of the risk posed by Irish-related terrorism was published for the first time "to encourage people to remain vigilant", she said.