Royal Wedding: Brits Worry Irish Terrorists May Target Kate and William

That device, ABC News has learned, appears to have been composed of just a few ounces of powerful plastic explosive placed in an aerosol can affixed under the officer's car.

The Irish terrorists have a long record of attempting attacks on the royal family.

Prince William's parents, Prince Charles and the late Lady Diana, were targeted by an IRA bomb plot in 1983 as they attended a London concert of Duran Duran.

In 1979, an IRA bomb killed Lord Mountbatten, the Queen's cousin and a mentor to Prince Charles.

The resurgent groups, while smaller in number and enjoying substantially less popular support, have similar ambitions, said Professor Paul Bew, a professor of politics at Queen's University in Belfast and a recognized expert on Irish dissident violence.

"It must be the ambition to repeat some of the dramatic explosions that happened, for example, in London during the IRA campaign," Bew told ABC News.

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"They are a much less potent force, but they can do it," he said. "And therefore everybody has to take on board the fact that there is something new in play here."

And Bew says because the groups, in fact, have less popular support, they may feel more emboldened in their targeting.

"They would not be constrained by a view which said, 'That's going too far,'" said Bew. "They're more of a wild card, they're more unpredictable."

The mortar tube is plastic, and the bombmaker has used the gas cylinder from a car airbag as the propellant. Earlier models required the mortar to be within 30 to 40 yards of the barracks or police station. Image obtained exclusively by ABC News.

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