Almost four million UK energy customers have been overcharged by their supplier due to billing errors in the last year, at a total of £270 million, a survey suggests.
Suppliers most commonly applied the wrong tariff or product details to an account - affecting 36% of overcharged customers, applied incorrect fees (31%), or set a charge based on a meter reading that differed from the one provided by the customer (27%), the poll for price comparison site uSwitch found.
Other errors - which left customers on average £72 out of pocket - included wrong direct debits, muddling bills up with someone else, charging twice for the same product and adding up totals incorrectly.
Some 14% of those who had been overcharged said a special offer or discount was not applied to their account.
Half (51%) waited more than a fortnight before the issue was resolved by the supplier, but almost one in 10 are still waiting, the poll found.
USwitch energy spokeswoman Claire Osborne said: "Consumers have a right to expect correct bills. It's unacceptable in this day and age that customers are picking up the cost of suppliers' mistakes.
"Households are already trying to cope with the high cost of energy and can't afford the additional cost of simple blunders.
"Accurate bills are essential if consumers stand any hope of taking control of their energy use and spend. Recent upgrades by some suppliers to billing systems have resulted in teething problems, but today's figures show there's still more for the industry to do.
"We urge customers to always check their bills carefully, and speak immediately to their supplier if they think they have been short-changed.
"Consumers should also always provide up to date meter readings to avoid estimated bills, and check that the figure they provided has been used."
:: Opinium surveyed 2,001 UK adults between between May 20-24.
A Department of Energy and Climate Change spokeswoman said: "We are on the side of bill-payers and it is our number one priority to ensure bills are kept as low as possible for families and businesses. But the real power is in the hands of the British people.
"If bill-payers are concerned about the deal they are getting from their supplier, we urge them to join the millions of others who are switching and saving."
Energy UK, which represents the industry, said: "Energy companies take accurate billing very seriously and where there are problems, the majority are resolved within 24 hours.
"Suppliers are actively working to improve billing for their customers. That is why companies covering 80% of the market have signed up to the Energy UK Billing Code to ensure greater accuracy of bills.
"If any customer is concerned or has a problem with their energy supplier they should speak to their company first."