An armed police officer speaks on his radio as he patrols near the Ministry of Defence in London, Britain May 11, 2016. [Photo/Agencies]

LONDON - Britain's internal security service, MI5, Wednesday increased the threat level of a mainland terror attack from moderate to substantial.

MI5 officials cited a threat of Northern Ireland-related terrorism for the change in threat-status. The warning means a terrorist attack in England, Scotland or Wales is now "a strong possibility".

"This means that a terrorist attack is a strong possibility and reflects the continuing threat from dissident republican activity," a MI5 spokesman in London said on Wednesday night.

"As a result of this change, we are working closely with the police and other relevant authorities to ensure appropriate security measures are in place."

The spokesman said the higher threat of Northern Ireland dissidents striking the mainland is still lower than the terrorism threat in Northern Ireland which remains at severe. The threat of an attack by international terrorists to the United Kingdom as a whole also remains unchanged at severe which means attacks are highly likely.

A "severe" threat warning is the second highest in the threat table.

"The main focus of violent dissident republican activity continues to be in Northern Ireland where they have targeted the brave police and prison officers who serve their communities day in and day out," British Home Secretary Theresa May said on Wednesday night.

There has not been an attack by Irish dissidents on the British mainland for 15 years.

Earlier this year a dissident group known as the New IRA was said to be responsible for the murder of a prison officer in Northern Ireland. The New IRA has issued a warning that its members were "determined to take the war to the age-old enemy of our nation".