A top energy expert has predicted the UK could face massive power cuts like that which has brought chaos to much of the eastern United States.
Professor Fells says UK power companies are losing money
Ian Fells, professor of energy conservation at Newcastle University, says power companies are losing money and are being forced to cut production to the minimum.
But the Department of Trade and Industry (DTI) said it was premature to start drawing comparisons between the US and UK systems.
Professor Fells' comments follow the biggest power failure in US history, which affected 50 million people and crippled cities from New York to Toronto.
Canadian officials blamed a fire at a power plant near Niagara in upstate New York for the outage.
US officials have disputed that, but insist terrorism was not to blame.
Professor Fells warned the UK could face similar failures in the event of a particularly cold winter.
He said the National Grid had admitted there would be "vulnerable weeks" in January and February, when it might not be able to guarantee electricity supplies.
Professor Fells said: "Last December we were within three minutes of power cuts.
"This is all because companies are now reluctant to have as much spare capacity as they used to.
"At one time there was about a 25% margin of spare capacity in the network.
"This is all because these companies are losing money."
The UK network is run by National Grid Transco, which also owns Canadian distribution company Niagara Mohawk, blamed by some for the US fault.
A spokeswoman refused to speculate about the likelihood of a similar event in the UK until the cause of the US blackout had been determined.
But she said Transco would prefer to have greater spare capacity than current levels, to cope with occasional high demand.
She questioned Professor Fells' 'three-minute' claim, but admitted: "We did have a tight time on December 10 last year."
She added that forecasts indicated they would have enough electricity to meet peak winter demand.
Niagara Mohawk has dismissed speculation that its system may caused the US fault as "uninformed and premature".
The DTI has promised an assessment to examine what implications the blackout could have for the UK system.
But they said the two energy networks were very different.
"Until we know what caused the US problems, these comparisons will be irrelevant," said a spokesperson.
"However, there are significant differences between the way UK and US energy networks operate."
Britain has an integrated transmission network regulated by a single body, Ofgem, while the US grid consists of a number of linked power company transmission systems over a larger geographical area.
The DTI added: "Contingencies have been studied and plans are in place here to cope with any major outages of the system.
"It is highly unlikely that a single fault could lead to the collapse of the whole system."
Former energy minister Brian Wilson said increased competition in the energy market had driven down the cost of electricity.
But he warned that meant there was no "over-capacity" of electricity to cope with blackouts.
He told BBC Radio 4's The World At One: "Electricity prices have to rise - so the political gain of driving down is eventually countered by the political opprobrium you get when they start to rise."