Britain is on heightened alert for an attack from Northern Ireland-related terrorism.

The security service MI5 raised the threat level to Great Britain from Northern Ireland yesterday from "moderate" to "substantial".

UK Home Secretary Theresa May said the change means a terrorist attack is a strong possibility, and reflects the continuing threat from dissident republican activity.

In a statement to the House of Commons yesterday, she said the British Home Office was working closely with police and other relevant authorities to ensure appropriate security measures are in place to deal with the raised threat.

The threat level to the UK from international terrorism remains unchanged.

The threat to the UK mainland from Northern Irish terrorism was last at substantial in 2011, before it was reduced to moderate in October 2012.

Belfast-based security correspondent Alan Murray analyses where this latest change is coming from.

"We are surmising that they are worried that the capability of making devices - which has been seen here with the death of a prison officer in March - is such that it could be transported to the area of Britain", he told Newstalk Breakfast.

"We were warned that probably someone would be killed over the Easter weekend, or there abouts, in relation to the commemoration of the Easter Rising".

"(The) reality was that nothing really happened - we had the parades, the marches, the balaclavas, the sunglasses and the camouflage outfits - but no one was killed".

"But civilians have been killed in Belfast in the nationalist community we believe by the dissidents in two instances in the last 25 days" he added.