Energy provider Scottish Power is the latest energy firm to announce inflation-busting price rises.
Scottish Power says it must pass on rising costs
Gas bills are to rise by an average of 15% and electricity prices by 14% from Saturday, the company said.
Scottish Power, which has 5.2 million customers, has blamed soaring coal and wholesale gas costs for the increase.
It becomes the fourth of the big energy providers to announce double-digit rises, after similar moves in recent weeks by British Gas, Npower and EDF.
Customers on dual fuel tariffs face a 14.8% rise - a £123 increase on an average annual household bill to £959, but the 1.2 million customers on fixed price deals are not affected.
Although there will be regional variations, Scottish Power predicts its customers' average annual household gas bill will rise by £79 to £604, and electricity bills should rise by £45 to £367 on average.
Willie MacDiarmid, Scottish Power's Director of Energy Retail, said: "We have worked hard to protect our customers from these increases for as long as possible and offer consistently competitive energy prices.
"Our input costs are now at record levels, having increased by up to 97% since February 2007. Like other energy providers, we are now forced to pass on some of these higher costs."
The industry is dominated by the 'big six' energy providers.
Rivals British Gas - the UK's biggest power provider - increased gas and electricity prices by 15% in January.
The same month, Npower raised its electricity prices by 12.7% and gas by 17.2% and EDF Energy also put up its electricity tariffs by 7.9% and gas bills by 12.9%.
German-owned E.On is expected to be the next to make an announcement on prices, while Scottish and Southern Energy has pledged to leave prices unchanged until the end of next month.
Mark Todd, director at energyhelpline.com, said: "As predicted, Scottish Power was the next major supplier to increase its prices.
"Most of Scottish Power's customers are in Scotland, North Wales and Merseyside. With snow coming this weekend, the last thing people need is bills soaring.
"However, there is no need to panic. There are massive savings to be had by switching to a cheaper tariff. In Scotland, switching to the cheapest tariff and paying by direct debit will reduce bills by £418 a year."
Scottish Power is based in Glasgow and owned by Spanish energy firm Iberdrola.