The U.K., Europe’s second-largest economy, risks power shortages and a jump in utility bills this winter because of repairs to aging nuclear reactors and pollution rules that limit coal burning. The nation’s electric grid cut forecasts for spare capacity today to a level where the loss of any one of its 38 largest power stations at a time of peak demand risks forcing factories to shut down to save energy, data from London-based network manager National Grid Plc show. The last time that happened, in May, wholesale electricity costs jumped 13 percent in a day.

U.K. power prices for this winter rose to a record today and are more than double a year ago after British Energy Group Plc closed two of its eight nuclear power stations and extended maintenance at others, while curbs on sulfur dioxide emissions forced coal-fed plants to cut operating hours. One of the reasons the government supported Electricite de France SA’s 12.5 billion-pound ($23 billion) agreement yesterday to buy British Energy was a pledge to build the first atomic plants since 1995.

National Grid said today that surplus power supplies this winter will be as little as 826 megawatts, or 1.5 percent of consumption, cutting the predicted minimum from 1,373 megawatts last week. The amount represents what’s left after the grid meets peak demand and maintains a margin to provide reliable service.  The estimates leave little flexibility. The grid’s forecast is based on information provided by the country’s electricity generators. Peak demand for the week of Nov. 10 is forecast at 56,500 megawatts.

November baseload power, the contract traded via energy brokers for around-the-clock deliveries, rose 8.3 percent to a record 124 pounds a megawatt hour today. Winter baseload was at a record 105.25 pounds a megawatt-hour, more than double the 45.90 pounds a megawatt-hour price for winter a year ago.

The forecast indicates that for now there’s enough generation available, said Stewart Larque, a Warwick, England- based spokesman for National Grid, before the most recent update to the forecast.

“The reason why it is published is so that people can make the right decisions,'’ he said. “If the figures were negative, that’s when it becomes a signal to the market to make more capacity available.'’

Electricity can’t be stored, so production needs to continually meet demand. The power grid has spare plants that only run at times when demand and prices are at their highest. Mothballed generators can return to service if the cushion threatens to disappear, Larque said. Supplies may push prices to “the sort of level that forces industrial demand off the system,'’ said Jeremy Nicholson, the director of the Energy Intensive Users Group, whose members include the steel and glass industries. “We are heading in that direction.'’

Day-ahead baseload power prices this month have reached 124.50 pounds a megawatt hour, the highest since March 2006, and more than three times costs the same time last year. The next- day contract traded at 99 pounds a megawatt hour today.

Sizewell Shutdowns

National Grid said on May 27, the day when the shutdown of British Energy’s 1,200-megawatt Sizewell-B power plant caused supply interruptions in parts of England, that steps to reduce demand were imminent because of insufficient spare supplies. It was the first such notice since 2006. This year it has warned five times that spare supplies aren’t adequate in addition to the demand-reduction notice.

Scottish & Southern Energy Plc and International Power Plc are upgrading plants to comply with restrictions on gas emissions. They may resume normal output in the coming months.  “Until that work has been completed, presumably there’s some risk,'’ Nicholson said.

British Energy plans to start four reactors in the fourth quarter that have been closed for a year because of corroded wires.

Those plants may be needed. Winter temperatures may be colder than normal, according to Jim Dales, senior forecaster at British Weather Services, which sells forecasts to businesses including energy companies.

November and December may be milder, while January and February may “deliver significantly below-average temperatures,'’ he said.