Gas and electricity suppliers are even worse than HM Revenue & Customs and council tax departments at getting bills wrong, according to new research.

Almost nine million households have had to deal with inaccurate bills from energy suppliers in the past two years. Even worse, when inaccuracies are spotted it takes, on average, two months to sort out, reveals the survey by the comparison firm uSwitch.com.

Energy companies are considered less accurate than council tax departments, mobile phone companies, the DVLA and the Revenue, according to the research.

The Revenue, which is under fire over plans to reduce the number of years taxpayers can claim back over payments from six to four years, came second in the league of shame while council tax departments were third.

The research shows 40 per cent of energy customers are owed money because of overcharging and 26 per cent are in the red to their supplier, making it difficult for them to switch to a better tariff.

Over 11 million energy customers have unexpectedly fallen into debt because of bills based on estimates that could be wrong. The average amount owed is £137, though more than one in ten owe between £201 and £500.

Npower was ranked the worst for customer satisfaction by the energy watchdog Energywatch last month. Although the overall number of consumers contacting Energywatch has fallen, queries regarding Npower have grown by 21 per cent in the six months to March. Most of the complaints related to direct selling, and problems with billing and switching supplies.

The German owned firm, which has 6.8million customers in Britain, made a £570m profit last year and signed up 170,000 new customers despite charging some of the highest prices for gas and electricity.

Suppliers say they are working with consumers to overcome these issues. EDF Energy has introduced ‘Read Reduce Reward', a scheme whereby it gives customers Nectar points for submitting their own meter readings.

Scottish and Southern Energy encourages customers to text in their readings and British Gas is shortly going to bring in a new SMS texting scheme.

All suppliers are also involved in smart meter trials that would allow energy firms to read customer meters without having to make a visit to their homes. E.ON, for example, is piloting smart meters as part of a two year, £12m investment in metering technologies.

Ann Robinson, director of consumer policy at uSwitch.com, comments: "Suppliers are making it easier than ever for customers to read and supply their own meter readings and in some cases are even rewarding those who do so. But whether there is a reward or not, I would still urge consumers to respond to meter reading requests or to supply their own readings.”