Britain has raised its threat level to substantial, the third highest on the scale designed to alert the population to the possibility of a terrorist attack.
The Home Office says there is a strong possibility that dissident Irish Republican groups - opposed to power-sharing in Northern Ireland - will attack the British mainland.
The Home Office confirmed it was the first time this threat level had been published, while Home Secretary Theresa May said it meant an attack was a "strong possibility".
Three years ago the security services concluded that the threat from terrorism in Northern Ireland was low and likely to decline further, the BBC reports.
Last week, however, MI5 Director-General Jonathan Evans acknowledged there had been a "persistent rise" in activity and ambition by dissidents in Northern Ireland over the past three years.
Two soldiers and a police officer were shot dead last year and since then attacks have continued on a fairly regular basis.
The BBC's Ireland correspondent says the number of active dissident Republican paramilitaries is small, but among them are some former Provisional IRA bombers who have become disillusioned with the peace process.