The Prime Minister should launch an urgent, independent review into the rising cost of household energy bills, consumer body Which? says.
In a letter to David Cameron after new price hikes by four of the Big Six left customers struggling with higher fuel bills, Which? executive director Richard Lloyd said the energy market was "broken".
A review was needed to look at rising prices and whether competition between suppliers could be made to work more effectively to help consumers, he urged.
With the average bill up 13% since a Government energy summit a year ago, "it is no wonder consumers tell us that energy prices are one of their top financial concerns", he said.
There was little evidence the Government was living up to its promise to make energy companies be competitive, with 75% of consumers on the most expensive tariff, and the numbers switching suppliers continuing to decline.
Mr Lloyd said people were questioning whether they were paying a fair price for gas and electricity, as companies blamed wholesale price rises for bill increases, but bills were not reduced when costs dropped.
The cost of implementing environmental and social policies, such as subsidies for renewable power and requirements that energy companies help people make their homes more energy efficient, was also being blamed for pushing up bills.
But Mr Lloyd said there was no hard evidence to back up that claim.
An urgent, independent expert review needed to look at whether the reasons given for recent price increases were justified.
It should also identify what reporting requirements should be demanded of energy companies on wholesale prices and social and environmental costs to increase transparency.