Centrica has warned that households will face higher energy bills due to rises in wholesale gas prices and costs.
British Gas owner Centrica dropped its standard electricity tariff by five per cent in January after big rises in both gas and electricity bills over the summer.
However it warned that the trend for retail energy costs "remains upwards", with wholesale gas prices 15 per cent higher for next winter and other costs set to add £50 to the cost of supplying the average household this year.
"Consumers will need clear evidence that price rises are warranted if they are to stomach further increases to their bills," said Audrey Gallacher, director of energy at Consumer Focus.
"People simply don't know whether what they are asked to pay is fair - the perception is that suppliers are quick to pass on high price rises and slow to pass on small price cuts.
"Much greater transparency on costs, pricing and profits is needed for customers to know whether they're getting a fair deal."
She called for the extra revenue the government receives from carbon taxes to be used to help the millions of people who are being hit hardest by rising costs.
A round of price reductions by the UK's major suppliers earlier this year were not enough to offset price hikes over previous months, and utility bills have been a driver of the rising cost of living that has squeezed households.
British Gas, which has 15.9 million residential customers, increased gas bills by 18 per cent and electricity by 16 per cent in August.
As well as the rise in wholesale gas prices, Centrica said it was battling with higher costs for transport and distribution, metering, green levies and social costs.
Centrica's warning came as it said its own trading this year was in line with expectations, as higher wholesale gas and power prices benefited its "upstream" operations.
This helped offset the effect of mild weather and energy-saving initiatives on its "downstream" business in the first quarter.
Cold and wet weather boosted demand in April, leaving average domestic gas consumption in the first four months of the year one per cent higher than a year ago, although electricity was down three per cent.
Residential customer numbers were broadly unchanged from the start of the year, helped by January's tariff cut, which it said re-established it as the cheapest major electricity supplier in Britain.
Its business customers' average gas and electricity consumption was down by one per cent and four per cent respectively, and its profit margins were under pressure amid the recession.
Tom Pering, an energy analyst at Inenco, said cost hikes were caused by a combination of issues, including supply security and the increasing cost of Government legislation.
"This increased cost will inevitably be passed on to residential and business customers in the form of higher bills," he added.
"However, the big six energy companies are still not doing enough to inform UK households of their cheapest tariffs and helping them reduce bills."
Centrica's shares were up one per cent after it said it was trading in line with expectations.
The City expects underlying operating profits to rise 20 per cent to £2.9 billion in 2012, driven by a 37 per cent rise in its upstream operations.
Its UK residential energy supply business is expected to see profits rise 13 per cent to £592 million.
"Hard-pressed families and pensioners struggling to make ends meet thanks to the recession made in Downing Street will be astonished that Centrica, who made billions in profits and paid out millions in bonuses last year, are now threatening another round of price hikes," said Shadow energy secretary Caroline Flint
"When wholesale prices rise, energy bills rise like a rocket. But when they fall, bills come down like a feather.
"Instead of letting the energy giants get away with trying to soften up their customers for bumper bills, the government must make the energy companies come clean about the price at which they buy and sell energy.
"For the long term, we need fundamental reform to make our energy market work in the public interest."