Britain has raised the threat level of an Irish-related terror attack from moderate to substantial, meaning a strike is a "strong possibility," the government said Friday.
It is the first time a threat level has been published in connection with the threat to Great Britain, meaning England, Wales and Scotland, from Irish militant groups.
The head of the U.K.'s MI5 security service warned last week that dissident Irish Republicans opposed to British rule in Northern Ireland could seek to strike mainland Britain, according to BBC News.
"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," the U.K.'s Home Secretary (interior minister) Theresa May said in a statement.
"Judgments are based on a broad range of factors, including the intent and capabilities of terrorist groups," she added.
'Remain vigilant'
"This is the first time we have published the Irish-related threat assessment to Great Britain," May said. "This is in the interests of transparency and to encourage people to remain vigilant."
She urged the public to report suspicious activity to the police and security services to help their "continuing efforts to discover, track and disrupt terrorist activity."
"The first and most important duty of government is the protection and security of the British people," May said.
"We have been consistent in stating that the threat to the U.K. from terrorism is real and serious. The balance we aim to strike is keeping people alert but not alarmed," she added.
The U.K. has five levels of threat: critical, meaning an attack is expected imminently; severe, an attack is highly likely; substantial, an attack is a strong possibility; moderate, an attack is possible but not likely; and low, an attack is unlikely.
The statement from the Home Office said people "should not let the fear of terrorism stop you from going about your day-to-day life as normal. Your risk of being caught up in a terrorist attack is very low."
A 1998 peace deal largely ended three decades of violence between predominantly Catholic Nationalist groups who want a united Ireland and mainly Protestant unionists who want Northern Ireland to remain part of the United Kingdom.
The main paramilitary organizations on both sides, such as the Provisional IRA, have surrendered their weapons, but militant splinter groups have stepped up attacks in Northern Ireland in recent months.
Dissidents have attacked security forces several times, with the Real IRA believed to be leading much of the campaign including car bombings and shooting at police officers.
