Britain has raised its terror alert from Irish dissidents, saying the threat has risen to a point where an attack on mainland Britain is "a strong possibility."
The Home Office says the change was made Friday from "moderate" to "substantial" - the middle rung on the government's five-point threat scale.
The threat from international terrorism, which is ranked on a separate scale, stands at "severe," meaning an attack is considered highly likely.
The highest level is "critical" - meaning an attack is imminent.
The government has never before released the threat assessment of Irish groups.
Home Secretary Theresa May said it was being made public "in the interests of transparency and to encourage people to remain vigilant."
Last week the head of the MI5 spy agency warned that dissidents who reject Northern Ireland's peace process could strike mainland British cities for the first time since 2001.
Jonathan Evans said Irish republican splinter groups, who having the British province of Northern Ireland join the Republic of Ireland, have access to weapons, including Semtex explosives, and funds from smuggling and drug trafficking.
Earlier this month, The Guardian newspaper quoted the dissident Real IRA group as saying it planned attacks in England and would focus on banks.
Republican dissidents last made a successful attack in England in August 2001, exploding a car bomb near a shopping center in west London, injuring 11 people.
Evans said dissidents had mounted or attempted 30 attacks this year in Northern Ireland - including a car bombing at MI5's base in the region, which caused no serious injuries - an increase from 20 attacks last year.