EDF Energy, the firm which owns London Energy, Seeboard and SWEB, is to raise the price of gas and electricity for its five million customers.
Gas prices are rising by nearly 15%
From 13 March, its gas prices will rise by 14.7% and electricity by 4.7%.
This will raise the average electricity bill by 30p a week, while the average gas bill will rise by £1.32 a week.
EDF is the latest in a long line of energy firms to raise bills, following a 100% increase in wholesale energy costs in the past year.
"With soaring wholesale energy costs, we have been forced to raise our prices," said Derek Lickorish, EDF energy spokesman.
"However, through ownership of our own generating capacity, we have been able to limit the overall rise for all our customers," he added.
In response to EDF's move, consumer watchdog Energywatch urged consumers to shop around for the cheapest supplier.
But the watchdog added that shopping around would only offer limited comfort for the most vulnerable in society.
"Average price rises are being continually pushed up and it won't be long before we see £1,000 energy bills as the norm," said Adam Scorer, director of campaigns at Energywatch.
"We are faced with a doubling of the number of households in fuel poverty who face the daily decision to eat or heat," he added.
Last week, Scottish Power, the UK's fifth biggest energy supplier announced rises of 8% for electricity and 15% for gas, and British Gas is reportedly planning to introduce large price rises.