Three coded bomb threats by dissident Irish terrorists have been called in to London police in the past 24 hours, ABC News has learned, prompting a public alert by Scotland Yard on the eve of Queen Elizabeth II's historic trip to Ireland.
None of the threats mentioned the Queen's trip specifically -- which will mark the first time a British monarch has stepped foot in the Republic of Ireland -- or President Obama's planned visit to London later this month, sources said. One threat mentioned a location, but no bomb was found in the area.
Scotland Yard acknowledged one of the threats and noted it was not specific in a bulletin posted on its website. The police force encouraged citizens to "continue to go about their business as usual but we encourage the public to remain vigilant and report any information about unusual activity or behaviour which may be terrorist-related..." Parts of central London close to Buckingham palace and government buildings were closed to traffic, according to British media reports.
The phoned-in threats are the latest in an aggressive campaign by Irish terrorists that has so far confined itself largely to attacks on police and security services in areas inside Northern Ireland that include Belfast, Omagh and Londonderry. An attack in London is an aspiration of the terrorists, British law enforcement and intelligence sources have told ABC News.
The threat level for Irish terrorism has not been raised in relation to the latest threats, Scotland Yard said, but "remains substantial meaning that an attack is a strong possibility." It was raised to substantial in September 2010.
According to senior British security officials, a small group of resurgent Irish terrorist has been responsible for more than 40 significant terror attacks over the last year. The British officials in charge of countering domestic terrorism, MI5, have recently increased surveillance of suspected Irish terror leaders and shifted resources from al Qaeda to the Irish groups.
Largely unnoticed outside of the United Kingdom, splinter groups of Irish Republican militants have increasingly ignored the peace agreement reached 13 years ago that was meant to end IRA violence.
ABC News' Lee Ferran and The Associated Press contributed to this report.