British Gas owner Centrica has announced it is to raise gas prices by 35% and electricity prices by 9%.
The UK's biggest domestic energy supplier said that the price hikes would take place with immediate effect.
It blamed "soaring wholesale energy prices", but added that standard tariff prices would not rise again in 2008.
The move comes just a few days after rival EDF Energy put up gas prices by 22% and electricity prices by 17%, with other firms expected to follow suit.
Watchdog Energywatch said it believed the 35% gas bill rise was the biggest single increase in the price of a utility seen to date.
Centrica said the average dual fuel bill for a British Gas customer would go up by 25% - putting the average household bill at about £1,250. This is the second increase this year, after a 15% rise in bills in January.
"We very much regret that we have had to make this decision at a time when many household budgets are already under pressure," said British Gas managing director Phil Bentley.
"The simple fact though is that we have entered an era of unprecedented high world energy prices."
Centrica said that wholesale gas prices in the coming winter would be up 89% on the previous winter.
A report prepared for Centrica earlier this month warned that annual average gas bills could rise from £600 to more than £1,000 early in the next decade.
It added that the UK was suffering from diminishing gas reserves, and estimated that the UK would import 40% of its gas this year compared with 27% last year.
The price of gas has risen in recent months as it is linked to the cost of oil, although oil prices have started to fall again in recent weeks.
British Gas, which has 15.9 million customers, said its profits for the first half of the year were down by 69% to £166m.
The UK's biggest domestic energy supplier said that the price hikes would take place with immediate effect.
It blamed "soaring wholesale energy prices", but added that standard tariff prices would not rise again in 2008.
The move comes just a few days after rival EDF Energy put up gas prices by 22% and electricity prices by 17%, with other firms expected to follow suit.
Watchdog Energywatch said it believed the 35% gas bill rise was the biggest single increase in the price of a utility seen to date.
Centrica said the average dual fuel bill for a British Gas customer would go up by 25% - putting the average household bill at about £1,250. This is the second increase this year, after a 15% rise in bills in January.
"We very much regret that we have had to make this decision at a time when many household budgets are already under pressure," said British Gas managing director Phil Bentley.
"The simple fact though is that we have entered an era of unprecedented high world energy prices."
Centrica said that wholesale gas prices in the coming winter would be up 89% on the previous winter.
A report prepared for Centrica earlier this month warned that annual average gas bills could rise from £600 to more than £1,000 early in the next decade.
It added that the UK was suffering from diminishing gas reserves, and estimated that the UK would import 40% of its gas this year compared with 27% last year.
The price of gas has risen in recent months as it is linked to the cost of oil, although oil prices have started to fall again in recent weeks.
British Gas, which has 15.9 million customers, said its profits for the first half of the year were down by 69% to £166m.