The National Grid says fears of winter power blackouts are unfounded, and denied that surplus capacity is low.
The energy trader Inenco says the UK could face power blackouts "within weeks" if a sudden cold snap or unplanned power outage occurs.
But the National Grid says it will be maintaining 4000MW of spare capacity throughout winter - a safety net equating to four large power stations.
Work on coal and nuclear plants is no threat to emergency capacity, it said.
Talk of blackouts has been escalating this week, since the National Grid published its latest forecast of power supply and demand over the winter months.
The update revealed that in November, predicted surplus capacity had dropped as low as 800MW - leading to newspaper reports that it would take just one power station failure to cause a blackout.
Dome
Blackouts could hit hard by time of London 2012, said Inenco
David Hunter, from energy consultants McKinnon and Clarke, told the BBC there was "real concern" that there could "a lack of enough power available for short periods".
"What's definitely true is that margins are tighter than they've been for a long time and not very much has to go wrong to turn that towards brownouts or blackouts," he said.
Price hike
The price of electricity rose above £121/MWh, and prices for November delivery rose 7% in a matter of hours.
Inenco, a company which procures energy for major industrial users, has said that the UK could face power blackouts "within weeks".
"The National Grid's forecast has such a low surplus in November, if there is a sudden spell of cold weather, or an unplanned power outage, we could easily see blackouts," said Michael Allen, an energy trader at Inenco.
"This is part of an ongoing problem with capacity. Our nuclear power generators are running at half their capacity, and what we have to replace them are sources which are either not working, in maintenance, or very expensive to run."
But according to the National Grid, these surplus figures are not the whole story.
The total "surplus" capacity also includes the standard 4000-5000MW "safety cushion" that the National Grid incorporates in its weekly targets.
The latest capacity forecast shows the UK is on target to achieve that 4000MW minimum, every week, for the remainder of the year.
Nonsense
"It is nonsense to suggest that the UK is at high risk of blackouts," said a spokesman for the National Grid.
"The margins of surplus capacity we are forecasting are similar to any other winter.
I just don't think it's wise to alarm people about this
Allan Asher, Energywatch
"If anything, forecasts have been worse in the past. We have published forecasts which predict a 'negative margin'.
"But when we do this, it prompts power generators to bring inactive plants back online in time to meet anticipated demand."
Goran Strbac, Professor of Electrical Energy Systems, at Imperial College London, said that blackouts are no more likely this winter than in any previous year.
"Where is the panic? I don't see any reason. The power generating companies do their homework very well and they will make sure their stations are ready to come online to meet the demand.
"I don't see any reason to doubt the National Grid's forecasts. They have no incentive to hide any potential shortages. In the past, they have often advertised them."
Allan Asher, chief executive of consumer watchdog Energywatch, told the BBC there were several periods this winter when "it's going to be very tight".
He blamed an "appalling lack of future planning and cavalier regulation", but said: "I just don't think it's wise to alarm people about this.
"Both at a regulatory and planning level a huge amount needs to be done, but that shouldn't be done with dire threats which overstate the concern now."