Households hoping to avoid future rises in energy bills by choosing a fixed tariff will find them almost 13pc more expensive than in the summer.
This is despite the fact that energy bills rose by an average of about 8pc following the recent round of price rises from the "Big Six" suppliers.
The cheapest fixed rate deal in August, before the price rises, would have meant an annual energy bill of £1,040 for the average household, according to Energyhelpline.com, the price comparison website. But the cheapest fix now costs £1,170, a rise of £130 or 12.5pc in the space of four months.
The cheapest variable tariff, by contrast, has risen by only £70 or 6.7pc from £1,050 to £1,120 over the same period, Energyhelpline said. The average bill has risen by £100 from £1,250 to £1,350, an increase of 8pc.
Mark Todd, a director of energyhelpline, said: "As switchers have flocked to fixed-rate tariffs, these rates have climbed in price dramatically. This shows what good value it was to fix early in the price rise cycle."
He said the typical energy bill for those who hadn't switched was a whopping £1,400. By switching, these families could often save up to £230 and get a rate guaranteed not to rise for the next two winters, he added.
An advisory body warned earlier this week that some 300,000 more homes were likely to have been pushed into "fuel poverty" by Christmas as energy prices soared.
The Fuel Poverty Advisory Group urged the Prime Minister to take stronger action to ensure that there was a more widespread and ambitious effort to tackle "spiralling" fuel poverty levels.
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