Four other people were seriously injured as drive-by gunmen fired two bursts of automatic fire outside the Massereene Barracks in Antrim - home to 38 Engineer Regiment - on Saturday night.

The victims were understood to be taking delivery of a pizza when the attackers pulled up in a vehicle and opened fire.

No-one has yet claimed responsibility for the shooting, but dissident republicans opposed to the peace process were immediately blamed.

It happened just 36 hours after Northern Ireland Chief Constable Sir Hugh Orde confirmed that undercover soldiers had been called in to carry out surveillance operations on dissidents amid warnings that the threat against his officers and military personnel was at its highest for almost a decade.

A car or van is understood to have pulled up outside the main gates of the barracks before 10pm. Soldiers and security staff thought pizzas were being delivered and walked straight into an ambush.

Secretary of State for Northern Ireland Shaun Woodward condemned the shooting as an "act of criminal barbarism". Downing Street added: "This is a terrible incident that we utterly condemn."

Northern Ireland's First Minister and Democratic Unionist Party leader Peter Robinson said he was postponing a trip to the United States as a result of the shootings. He said they were a "terrible reminder of the events of the past".

Ian Paisley Jr, a member of the Policing Board, said: "For the last 10 years, people believed things like this happened in foreign countries, places like Basra. Unfortunately it has returned to our doorstep."

Security chiefs had warned for months that dissident republicans were determined to inflict fatalities. Five police officers were wounded in separate gun and bomb incidents. There have also been a series of failed bomb attacks, one just a few weeks ago.