One of the "big six" energy suppliers has fuelled questions over the fairness of its prices after it revealed a jump in profits despite a drop in sales.
French utility giant EDF, which supplies electricity and gas to around 5.5 million residential and business customers, reported underlying earnings of £1.6 billion in the UK in 2011, up from £1.4 billion the previous year.
However, revenues were down to 8.6 billion euro (£7.1 billion) in the UK in 2011, from 9.4 billion euro (£7.7 billion) the previous year, as households used less gas over the milder winter.
EDF said underlying earnings were boosted by increased nuclear output and a drop in wholesale gas prices, but consumer groups said this would leave customers confused.
Audrey Gallacher, director of energy at Consumer Focus, said: "EDF Energy's UK profits have risen despite lower energy use in the milder winter. This will leave many customers wondering whether energy prices can, and should, be cut further."
She went on: "We need successful, profitable companies. But consumers need to know big profit margins are not being made needlessly at their expense."
EDF cut its gas bills by 5% from February 7 after wholesale costs fell by 9% and sparked a series of reductions by rival suppliers. However, the cut came after EDF increased gas tariffs by 15.4% in November.
Which? executive director Richard Lloyd said: "When people see energy suppliers announcing increased profits despite a mild winter, they're bound to question whether they're paying a fair price."
He added: "The energy companies must raise their game and demonstrate their commitment to giving consumers the best price for their energy. If they do, it could be the start of a better relationship with their customers."
The results come a week before British Gas owner Centrica publishes its own set of annual figures, in which it is expected to reveal a 4% increase in adjusted operating profits to £2.5 billion. However, the company is expected to reveal a 25% drop in British Gas profits to £550 million, which would be the lowest level in three years.