The attacks involving two huge lorry-bombs and booby-trap car mines was foiled shortly before the operation was due to go live, police believe.
Ten tonnes of explosive was seized during a series of dawn raids in which one terrorist suspect was shot dead and five other men were arrested.
Anti-terrorist officers believe the IRA planned to launch a series of attacks, almost certainly in London, that included two massive lorry- bombs similar to the Manchester and Docklands devices and smaller explosives aimed at politicians or public figures. Scotland Yard said they believed the bombings were probably planned for either yesterday or today.
The finds appear to have dashed any hopes that the paramilitaries might be preparing to call a new ceasefire.
John Major, while declaring himself "absolutely delighted" by yesterday's operation, said it once again put a question mark over the paramilitaries' commitment to the peace process. "It remains impossible to reconcile Sinn Fein's rhetoric for peace with the IRA's preparations for murder," he said.
Sources warned yesterday that the IRA could be expected to carry out a quick counter-attack to prove it still had the power to hit targets on the British mainland.
Yesterday's breakthrough came in a series of 4am raids on three properties in London and one in Sussex. Scotland Yard's Anti-Terrorist Branch retrieved 10 tonnes of home-made explosive, some of which was boxed and ready for priming, 2lb of Semtex, two booby-trap bombs for fitting under cars, 13 timer-power units, three Kalashnikov rifles, two handguns and ammunition. Two lorries, which are believed to have come from Northern Ireland, and a number of cars were also recovered. Most of the haul was from an industrial warehouse in Hornsey Vale, north London.
During a raid on a three-storey house in Hammersmith, west London, police shot a suspect who died later in hospital.
Local residents later described how armed police had approached the house shortly before dawn. Officers wearing flak-jackets and carrying ladders attempted to gain access to the building but became involved in a gun battle with two men inside the house. It is understood the property is a popular boarding house for Irish labourers working in London.
Gulapshan Ali, 32, a mother-of-two, said she was awoken by screams and shouting. "It was 4.30am and there was an awful banging and screaming. I heard the sound of breaking glass and then I heard what I knew were gunshots.
"My husband looked out of the window and saw about a dozen special policemen wearing bullet-proof jackets.
"They were in position crouching behind trees and cars surrounding the house as others seemed to be trying to break into the basement and used ladders to get in. One was shouting `throw down your weapon we are the police'.
"There were about eight or 10 armed policemen at this time. Then there was shooting which lasted for about 10 seconds. Then we heard one policeman say `I don't think he's got a chance'," she added.
Nearby were yesterday sealed off as forensic science experts began searches inside.
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