The energy company Npower is raising its prices for domestic customers for the second time this year, blaming rises in wholesale prices.
Bills will rise again for six million customers
Electricity and gas prices are set to rise by an average of 13.4% and 15% respectively, Npower has announced.
This follows its price rises of 13.6% for electricity and 14.5% for gas which took place in January.
The latest blow to consumers comes hot on the heels of Powergen and British Gas, who also recently raised prices.
"This is yet another bleak day for those who are already struggling to pay their energy bills," said Anne Robinson, director of consumer policy at uSwitch.com.
"While it was inevitable that Npower would raise their prices again this year, we are surprised and disappointed to see them doing this so soon after their last increase took effect."
Twenty-two million UK households have seen their energy bills increase since the start of 2006, according to uSwitch.com.
Npower's move follows this year's 22% gas and electricity price increases from British Gas and Powergen's increases of up to 24.4%.
Rival energy suppliers Scottish & Southern Energy, Scottish Power and EDF have also announced or implemented price rises already this year.
For the average Npower gas and electricity customer, the company's latest rise will spell an increase of £9.50 per month on energy bills.
"These changes follow massive increases in the cost of wholesale gas, which has trebled in price since 2003," said Npower managing director Kevin Miles.
Although this is the second time that Npower's prices have gone up this year, the company did not increase its prices in 2005, which may explain the need for this second rise, said website SimplySwitch.com.
"Despite this, their bills have still gone up by 53% for gas and 44% for electricity since January 2004," said Alistair Tillen at SimplySwitch.
Npower is the biggest supplier of electricity in the UK and one of the country's top three energy suppliers, with six million customers.