Suspicions that dissidents may use Christmas to wreck this month's agreement on implementing the Good Friday Agreement have led to top-level briefings with up to 30 organisations that could be targets.
Anti-terrorist officers from Scotland Yard and MI5 believe that the level of threat from breakaway groups opposed to the peace process is at its highest for some time. Such groups are recognised as being capable of unleashing attacks on the mainland, ranging from lorry bombs to shootings and incendiary attacks.
Police believe that some of the Provisional IRA's most experienced bomb- makers and logistics experts are among a hard core of some 200 dissidents in a coalition of splinter groups including the Real IRA, the Continuity IRA, INLA and renegade Provisionals. The Real IRA is believed to have planted the bomb in Omagh last year which killed 28 people.
Police sources say the security alert is the most serious for 18 months. It is perceived as being second only to that of an active bombing campaign. The security forces have already briefed likely targets for assassination, including senior military and political figures. More police officers are to be deployed on the streets of London and security has been stepped up in Docklands and the City.
The warning came as Peter Mandelson, Secretary of State for Northern Ireland, told MPs that although he believed the Good Friday Agreement could be implemented in full, he would freeze the process if the IRA did not deliver on arms decommissioning.
Mandelson warning, page 4