Anjem Choudary, a London-based radical cleric and leader of the now-banned Islam4UK group, said that the royal wedding would "certainly be a target for those who want to cause havoc in Britain," including Al Qaeda.
"The Queen and her children are actively engaged in the war against some Muslims in Iraq and Afghanistan," he told ABC News. "She has actively supported the role of Britain in its policy of occupation and slaughter in her Queen's Speech, and we can see that William himself has been involved in military action in Muslim countries."
One British Muslim group linked to Choudary, Muslims Against Crusades, posted a statement on its website criticizing the royals along with a countdown clock to the wedding day.
"Unfortunately, Britain's continued interference in Muslim lands is showing no signs of abating," reads the statement. "In light of this, sincere Muslims have decided to organize a forceful demonstration."
The statement goes on to call for Prince William to "withdraw from the crusader British military," promising a "nightmare" on his wedding day should he refuse.
Though a spokesman for Muslims Against Crusades told ABC News the group is non-violent, officials said they are more concerned the vague threat could spurn violent action by other, "self-radicalized" Muslims.
Resurgence of Irish Republican Terrorism
In addition to relatively new terror threats, British authorities say they are increasingly contending with the resurgence of a decades-old foe: Irish Republican terrorism.
Irish Republicans who reject the peace deal between the Provisional IRA and the British government are believed to have been behind at least 40 significant terror attacks against national security targets in 2010. Last fall, England raised the threat of attacks by IRA splinter groups from "moderate" to "substantial."
British authorities admit they underestimated the Irish. As recently as 2007, they had viewed the terror groups as a violent handful with little to no political support.
"At that point," said Jonathan Evans, director general of MI5, Britain's counterintelligence service, in a 2010 report, "our working assumption was that the residual threat from terrorism in Northern Ireland was low and likely to decline further as time went on and as the new constitutional arrangements there took root."
"Sadly, that has not proved to be the case," said Evans. "On the contrary, we have seen a persistent rise in terrorist activity and ambition in Northern Ireland over the last three years."
Finally, authorities are also preparing for possible disruption by English anarchists including the "Black Bloc" youth who have plagued organizers of events across Europe and the U.S.