Simon Hughes, the Liberal Democrat energy spokesman, called this week on the Government to give Ofgem, the energy regulator, more teeth and to refer the energy industry to the Competition Commission.

During parliamentary questions on energy and climate change on Thursday, Mr Hughes, MP for North Southwark and Bermondsey, said: “Given that energy ministers, in the past, and Ofgem, up to now, have been so feeble in dealing with the abuses by the big energy companies, will the Minister now commit that we will have the opportunity to have an energy Bill in the next session [of Parliament] where we can give real tough powers to Ofgem, and secondly, would he consider referring the energy industry to the Competition Commission?”

His call follows revelations in Times Money last Saturday that npower may have overcharged its 2.2 million gas customers by more than £100 million.

Consumer Focus, the consumer champion, has conducted an investigation, which shows that between April 2007 and April 2008, npower breached the terms and conditions of its energy licence, leading to potential overcharging of many customers.

The findings of the Consumer Focus study represent a severe embarrassment for Ofgem, the industry regulator, which had earlier investigated the same problem and failed to establish if any overcharging had taken place.

So far, npower has refused to say how many of its gas customers were charged more than the stated maximum of 4,572 higher-rate units in the 12 months from April 1, 2007. But Andy Beck, of Teignmouth, Devon, who won compensation of £1,300 after being overcharged by npower, says that it is highly likely that almost all were affected.

He says: “The industry’s definition of average consumption is 20,500 units, or kilowatt hours, per year. This is more than four times the 4,572 figure, so all but the very lowest users of gas would consume more than 4,572 units and potentially be victims of overcharging.”

He adds: “Npower has always refused to say how many higher-rate units it charged most of its customers between April 2007 and April 2008, but it is possible to make a pretty accurate estimate using information about its charging structure. In April 2007, npower charged 448 units at the higher rate, followed by six months from May to October at a flat rate of 381 a month, making 2,286 units.

“It then charged a much heavier level of higher rate units, 882, in the winter months of November, December, January and February — a total of 3,528 — followed by 272 in March. This makes a total, in that 12 months, of 6,534 units — 1,962 more than the maximum 4,572 stated in npower’s contract.”

A spokesman for Ofgem said: “We probed the energy market last year and have produced a package of remedies to better protect customers, which are now in place.”