Two relatively minor technical glitches within two minutes of each other triggered the most serious disruption to Britain’s energy supply network in more than 20 years this week.

But power industry insiders said that the shutdown, which led to blackouts in as many as 500,000 homes across the country on Tuesday, could have been even worse. “We could very easily have lost the whole system and it could have taken several days for it all to come back on,” one said.

Last night Ofgem, the power industry watchdog, threatened to start a full investigation. It said it had demanded a report from National Grid and would then decide how to proceed.

Ofgem was responding to claims that National Grid should have had sufficient spare capacity on hand to meet the loss in supply which followed unexpected shutdowns from two of Britain’s biggest power stations.

The last time a supply disruption took place on this scale was after the Great Storm of 1987.

The blackouts, which emphasised the growing instability of Britain’s crumbling power infrastructure, began on Tuesday morning at the Longannet coal-fired plant in Fife, Scotland’s largest power station, where all four units had been taken out of service by Scottish Power several weeks ago for maintenance.

One unit was brought back into service at 6am and was running at half its regular capacity - around 350 megawatts - when its operators detected a minor problem with an internal auxiliary power unit. When they decided to cut the plant to rectify this just before midday, a series of automatic relays kicked in to balance the impact across the national grid.

On its own, this should not have caused a problem, but just two minutes later in a completely unrelated, fluke incident, Britain’s most modern nuclear power station at Sizewell B on the Suffolk coast ran into trouble. “We had the first trip-out on a reactor in 3½ years,” Bill Coley, chief executive of the operator, British Energy, said.

He said that a faulty instrument had registered high steam flow from one generator, probably incorrectly. This had triggered an automatic shutdown of the entire plant because of inbuilt safety features.

The loss of Sizewell, which generates 1,200 megawatts of electricity, was a much more serious event but the combined loss of two units at opposite ends of the country, in such rapid succession, created a sudden, severe shortfall across the network. As automatic relays kicked in to try to balance power supplies, customers across the country started to suffer a series of rolling blackouts.

One of the first calls made by the controllers at National Grid from their headquarters in Berkshire was to Anglesey Aluminium, Britain’s biggest single consumer of electricity.

The plant on the outskirts of Holyhead requires a continuous 250 mega-watt power supply to operate its smelters. It was asked whether supplies could be temporarily cut in order to help relieve pressure on the grid nationally.

Under the terms of its contract, Anglesey Aluminium - along with a number of other major British consumers, mostly in the chemicals and steel industries - are paid to accept occasional supply cuts to assist in times of unexpected peak demand.

But these measures and the extra reserve generating capacity that was immediately on hand were simply insufficient to meet demand so National Grid issued two separate urgent warnings to power companies requesting they bring on more supply - and also reduce total voltage to customers.

Known in the industry as “going to low volts”, this second approach effectively slightly reduced electricity to, and dimmed the lights in, every home in the country.

Despite these efforts, in the hours that followed the network was plunged into chaos as a string of other generating stations suffered related problems, further eating into the cushion of spare capacity.

Power stations on South Humberside, Cottam and Deeside were all caught up in the problems, which were amplified because a run of cold and wet weather had led to a surge in demand while a number of other power plants were out of service as part of planned summer maintenance programmes.

Just after 4pm, National Grid issued a Demand Control Imminent warning - the second most severe warning it can give - calling on suppliers to bring on more generating capacity, gradually restoring normal service.

Nevertheless, some industry insiders asked why National Grid did not have sufficient reserve capacity to deal with the situation. The grid is legally required to ensure that 2,000 megawatts of extra capacity is kept on stand-by at all times.“Given that the total initial loss was only 1,510 megawatts, there should have been more than enough to deal with this,” one power industry source said.

National Grid rejected these allegations, saying that it was the victim of knock-on effects resulting from “exceptional circumstances”. A spokesman insisted that it did have the legally required minimum spare capacity.

Meanwhile, Alan Duncan, the Shadow Energy Minister, told The Times that the Government should hold an urgent review of electricity supply and provide the public with a full account of what happened.

“We need a full investigation into these power cuts which caused widespread shortages. Energy security is becoming an increasingly important issue and this unexpected tripping of the system needs a full explanation,” he said.

Nevertheless, the incident has focused attention on the need for further investment in Britain’s energy network. “It concentrates minds,” said one source. “The older your kit gets the more likely this is to happen.”

Where the sparks flew

— Power cuts were reported in London, Cheshire, Merseyside, East Anglia and Teesside

— The town centre of Louth, in Lincolnshire, surrounding villages and Market Rasen were all affected by the power cut. Three sets of traffic lights went out, causing traffic chaos in the town

— In Streatham, South London, at least five sets of traffic lights were left without power, causing two-mile tailbacks

— In nearby Clapham Junction, trains were affected for an hour before returning to normal

— Businesses in the Merseyside districts of Wallasey, Birkenhead, Ellesmere Port and Runcorn were also hit at around 11.30am

— In Tyneside, computers in dozens of companies went down, prompting comments on the internet about the fragility of the national grid

— Operations were cancelled after a hospital generator failed to work in High Wycombe, Buckinghamshire. Patients waiting for surgery at Wycombe General Hospital were turned away after fire broke out in one of the emergency generators

— Cleveland and Co Durham fire brigades responded to automatic alarms set off by the failure

— A power surge at North Yorkshire Police’s HQ near Northallerton caused a fire. Hundreds of staff were evacuated

— Eight people were rescued from a lift at Whinney Banks library, Middlesbrough

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