British Gas has warned that its gas prices are likely to increase by around 15% over the coming year.
British Gas has lost 445,000 customers since the start of 2005
The company's owner, Centrica, said its electricity bills would also rise. The price rises were a "necessity" due to soaring wholesale prices, it said.
Last year, British Gas increased its prices by 5.9% in January and 12.4% in September.
Centrica also warned of lower profits at British Gas, due to a continuing loss of customers.
It said British Gas' profits for the first half of 2005 would be less than the £274m reported a year ago after losing 445,000 customers since January.
More than 1.1 million customers quit British Gas last year, with many unhappy at the price rises.
Centrica's finance director Phil Bentley said a 15% gas price hike was the "sort of ballpark" it expected to make.
"We're talking about price rises that no-one has seen or done before," he added.
"We thought 12% was pretty racy last year and it's fair to say it will be more than that [this time]."
His comments came in response to an article in the Daily Mirror newspaper on Wednesday, that first suggested British Gas was planning additional price increases.
Defending the expected price rise, Centrica said wholesale gas prices were now expected to be 51% higher than a year ago.
Centrica added that while British Gas was continuing to lose customers, the flow had slowed to around 10,000 a week since May, compared to more than 22,000 a week previously.