The wedding next week of Prince William and Kate Middleton presents a tempting target for a small group of resurgent Irish terrorists responsible for more than 40 significant terror attacks over the last year, senior British security officials tell ABC News.
While there is no specific threat of any organized plot against the wedding, officials say there is "concrete intelligence" that one Irish group, calling itself the Real IRA, has been trying to move its operations beyond Northern Ireland to London.
"It is fair to assume that people are worried about an attack on the mainland," said Mark Hamilton, chief superintendent of the policed service in Northern Ireland.
The British officials in charge of countering domestic terrorism, MI5, have recently increased surveillance of suspected Irish terror leaders, shifted resources from al Qaeda to the Irish groups and raised the threat level of Irish-related terrorism from moderate to substantial.
"There is a massive effort by the intelligence services and the police to track these people down," said superintendent Hamilton.
Largely unnoticed outside of the United Kingdom, splinter groups of Irish Republican militants have increasingly ignored the peace agreement reached 13 years ago this week that was meant to end IRA violence.
"I think these people would have no compunction at all about carry out an attack, if they could, on an event like the royal wedding," said Professor Martyn Frampton, a history lecturer at Queen Mary, University of London, and the author of a new report, "The Return of the Militants: Violent Dissident Republicanism."
"Attacks on the mainland, particularly here in London, I think, remain the holy grail for these organizations," said Frampton in an interview broadcast Monday on Good Morning America.
"They would say it is an opportunity to target the British establishment," Frampton added.
Security officials say there are several splinter Irish groups that continue to target police officers and other targets that symbolize their perceived enemy: British rule of Northern Ireland.
The Arming Switch. Irish terror cells traditionally use an assembly-line method to manufacture bombs, timers, and mortars, so authorities fear there are at least a couple of other working devices out there ready for use. Exclusive pictures of potential devices were obtained by ABC News.
Security officials in England and Ireland tell ABC News they believe there are about 600 members of the real IRA and related splinter groups including the Continuity Irish Revolutionary Army.
The groups have demonstrated an ability to gather powerful weapons and have displayed a surprising skill in making new kinds of bombs, officials told ABC News.
Most troubling, officials say, is the use of rocket or mortar launchers capable of sending an explosive device of 40 pounds over security walls more than 300 yards in the distance.
Police have seized large quantities of high explosive and weapons in recent raids, according to police superintendent Hamilton.
"When people have that type of weaponry in communities, then the intention clearly is to cause devastation to somebody," he said.