An attack on the British mainland from the New IRA or other Republican terrorists is a "strong possibility", MI5 has warned as the threat level was raised.

The risk to Britain from Northern Irish terrorism has been increased from "moderate" to "substantial" and security is now being stepped up, Theresa May, the Home Secretary announced.

She said the move, recommended by the Security Service "reflects the continuing threat from dissident Republican activity".

Republican militants have been increasing their attacks in Northern Ireland in recent years and become more ambitious.

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Security sources have regularly warned that a desire to attack the mainland has always remained.

It is understood the fresh assessment is a result of concerns about the increasing capabilities of the dissident groups and their growing desire to attack the mainland.

In December the Government had warned Republican terror groups were capable of carrying out "one-off" attacks on the British mainland.

They are also sufficiently armed to pose an "enduring threat" and still "aspire" to target the UK, according to the National Security Strategy.

Andrew Parker, the Director General of MI5 also warned that the threat from such groups is four times greater than figures suggest and some are reverting to old Provisional IRA methods.

For every dissident attack in Northern Ireland the Security Service foil three to four, it is understood.

There are a number of dissident Republican factions including the New IRA, which was behind the murder of prison officer Adrian Ismay in March.

At Easter, the group warned it was "determined to take the war to the age-old enemy of our nation".

In a written statement announcing the increased threat level, May said: "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 general threat level to the UK from all forms of terrorism, including Islamist fanatics, is already at "severe" meaning an attack is "highly likely".

The threat level for Northern Ireland-related terrorism was last raised from moderate to substantial in 2010 and scaled back again two years later.

In October, a review for the Government concluded that all the main republican and loyalist paramilitary terrorist groups remain in existence, 17 years after the Good Friday peace deal.

The Provisional IRA's ruling body, the Army Council, still existed, but in a "much reduced form", MPs were told.

The main focus of violent dissident republican activity continues to be in Northern Ireland.

May said: "The reality is that they command little support. They do not represent the views or wishes of the vast majority of people, both in Northern Ireland and the Republic of Ireland, who decisively expressed their desire for peace in the 1998 Belfast Agreement and have been transforming Northern Ireland ever since."