Britain has raised the threat level of a dissident republican terror attack from moderate to substantial, the third highest level, meaning such an event is a "strong possibility".
Home secretary Theresa May today said it was the first time the government had published the threat level from Irish-related groups.
The threat from international terrorism remains "severe", the second highest level, which means the security services think an attack is highly likely.
"We have been consistent in stating that the threat to the UK from terrorism is real and serious," Ms May said in a statement. "The balance we aim to strike is keeping people alert but not alarmed. I would urge the public to report any suspicious activity to the police."
The government said the decision to raise the threat level was based on factors including the "intent and capability of terrorist groups". Ms May did not mention any specific threats and a home office spokesman would not comment.
The head of the MI5 security service warned last week that dissident Irish republican groups could seek to strike mainland Britain. Jonathan Evans said there had been a "persistent rise" in "activity and ambition" by dissident groups in the North over the past three years.
Since the start of the year, there had been more than 30 attacks or attempted attacks on national security targets by dissident republicans, compared with just over 20 for the whole of last year, he said.
He said there were increasing signs of co-operation and co-ordination between the various groups, deploying a greater variety of attack techniques with improved weapons capability, including Semtex explosives.
Agencies