Dissident Irish republican terrorists have issued a bomb threat for central London, police said, ahead of Queen Elizabeth II's ground breaking but sensitive visit to Ireland.

Three coded bomb threats have been called in to London police in the past 24 hours. Scotland Yard are investigating reports that the telephone threats were made using recognised code words but was not specific with regard to either location or time.

Officers are already on high alert over the "severe" terrorist threat from al-Qaida-inspired violence. The threat level from Irish terrorism remains at substantial.