Home Secretary Theresa May said it meant an attack was a "strong possibility".

It was the first time this threat level had been published, the Home Office confirmed.

The head of security service MI5 warned last week that dissident republicans opposed to British rule in Northern Ireland could strike mainland Britain.

The threat from Irish-related terrorism to Britain is still lower than the overall threat from international terrorism, which remains unchanged at "severe".

The new category of threat from Ireland applies to England, Wales and Scotland but not Northern Ireland, where the level also remains "severe".

Mrs 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.

"Judgements are based on a broad range of factors, including the intent and capabilities of terrorist groups."

She said the information was being released to encourage people to remain vigilant, and a balance needed to be struck between keeping people alert and not alarming them.