British Gas (BG) raised its prices by 12 per cent today, rendering it the most expensive UK supplier.
Owner Centrica informed its 16.5 million customers in July that gas bills would rise by 12.4 per cent and electricity bills by 9.4 per cent from September 4th.
BG raised energy prices by 22 per cent in March.
Pressure group energywatch said today's rise will make BG the most expensive supplier in the country, with the average annual gas bill rising to £707 and the average electricity bill up to £428 since 2003.
BG says wholesale gas prices and problems with supplies from Europe have led to higher tariffs. But it is nearly a fifth more expensive than other main providers for gas, and 17 per cent more for electricity.
In view of the price increases, Allan Asher, chief executive of energywatch, warned benefits should be better geared to help the elderly and disadvantaged.
Meanwhile Ann Robinson, director of consumer policy at services watchdog uSwitch.com, condemned BG's price rises as being unfair on the consumer.
"While most households have been left calculating how they are going to afford their energy bills after being subjected to record price rises over the last 12 months, British Gas has been busy calculating how it can get away with excessive price rises and an unjustified premium on many of its products," she said.
"The sad thing is that British Gas's tactics appear to be working, with 69 per cent of their customers apparently taken in by the company’s marketing strategy and ruling out a switch to an alternative supplier."
The government says a number of measures have been put in place to tackle fuel poverty, which many fear could result from further energy price rises.
UK households now pay 87 per cent more for gas and 56 per cent more for electricity than they did in January 2003. The reliance on price fluctuations in the global market will grow in the future, as imported gas is expected to grow to 50 per cent within four years and to 85 per cent by 2020.