The National Grid was left carrying the can today for unprecedented power cuts that left hundreds of thousands of people without electricity around Britain yesterday.

The transmission network blamed the blackouts on a sudden loss of frequency caused by the near-simultaneous failure of the Sizewell B nuclear power station and the Longannet 1 coal-powered fire station in Fife, both of which "tripped" within a couple of minutes of each other at around 11.30am.

Sizewell B is the UK's only pressurised water reactor station and produces almost 1,200 megawatts of electricity, or 2 per cent of the country's peak demand. Longannet can produce a massive 2,600 megawatts. The National Grid described the coincidental shutdowns as a "freak event" and pointed out that power had been restored to almost all customers within 40 minutes.

Industry sources were quick to point out today that of Longannet's four turbines, two have been switched off for over a year for refurbishment and the other two have been shut for maintenance in recent weeks. A Scottish Power spokesman confirmed that one was powered up yesterday morning before it was "tripped" at 11.30am, but it had only been producing about 350 megawatts of power when it was shut by a minor technical fault.

"Sizewell B going down is a significant event but Longannet shutting down was not. It should not have had the impact it did," one industry source said. "Somebody dropped the ball at the National Grid."

British Energy, the country's main nuclear power generator, was today firing Sizewell B back up. It blamed the plant's first unscheduled shutdown in more than three years on a faulty instrument reading.

The shutdowns prompted the National Grid to disrupt supplies to electricity distributors to protect the integrity of its network. There were further unexpected plant shutdowns later in the day which prompted urgent appeals from the National Grid for more generating units to be made available.

That produced isolated blackouts around the country. At Wycombe Hospital in High Wycombe, Buckinghamshire, operations had to be cancelled after an emergency back-up generator failed.

In and around Market Rasen and Louth, in Lincolnshire, some 23,000 homes were affected and there were also widespread power cuts on the Wirral, in Watford and in South London.

But one industry source pointed out that the National Grid is meant to operate with a 20-per cent margin, meaning that if electricity supplies drop it can immediately call for new capacity, first of all from units that are already switched on. The source added that the Grid did not issue any "notice of insufficient margin" until yesterday's powercuts had already happened, even though there must have been supply shortages beforehand.

Whether yesterday's power cuts turn out to be a management issue or due to infrastructure problems and constraints within the network remains to be seen, although everything appeared to be back to normal today.

The blackouts prompted warnings about the UK's "crumbling" energy infrastructure from McKinnon and Clarke, an independent energy consultancy, but the National Grid denied any systemic failures.

Stuart Larque, its spokesman, said: “We have a very robust system in the UK. It rarely fails and that’s why everybody is talking about it so much...It was just such a freak event."

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