The terror threat of an Irish-related attack in Great Britain has been raised from moderate to substantial. Essentially this means that the British government considers that an attack is a "strong possibility."

This is the first time in history a threat level has been published, in connection with Great Britain and Irish militant groups.

Last week the head of the MI5 security service said that Irish Republicans who were opposed to British role in Northern Ireland could strike the British mainland according to the BBC.

In a statement, U.K.'s Home Secretary (interior minister) Theresa 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.

"This is the first time we have published the Irish-related threat assessment to Great Britain … This is in the interests of transparency and to encourage people to remain vigilant."

She asked the public to report anything suspicious they noticed in their everyday lives and to help the police and security services in 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," said May.

"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," added May.

A 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."

In recent months militant splinter groups of the IRA have stepped up their attacks in Northern Ireland. They have attacked security forces and the Real IRA is believed to be leading much of the campaign including car bombs and shooting at police officers.