More than 3 million gas and electricity consumers have been overcharged by their supplier in the last year due to errors on their bills, costing them an average of £78 each, according to a study published on Friday.

Of the big six suppliers, Scottish Power is the worst offender, according to research by the comparison site uSwitch.com, which found 16% of the Glasgow-based firm’s customers had been sent an incorrect bill in the past year. The company has been beset by customer service troubles since introducing a new IT system for billing last year.

Bills from E.ON and EDF Energy contain the fewest number of errors, according to the research, although 8% of their customers still said they had been overcharged.

Of the energy customers who had a billing problem, 45% said the problem had been caused by a different meter reading being entered on the system to the one supplied.

Two in five complainants waited for more than a fortnight before the issue was resolved by the supplier, while 10% of consumers who were overcharged as a result of a mistake still haven’t received the money back from their supplier, they said.

Energy bill complaints have dominated the Guardian and other newspapers’ postbags in the last year, but little has been done to resolve the problem.

Tom Lyon, energy expert at uSwitch.com, said: “It’s unacceptable that customers are picking up the cost of suppliers’ mistakes. Households are already facing sky-high energy bills and simply can’t afford simple blunders.

“Correct bills are essential for people to stay in control of their energy and are the least customers should expect. Recent upgrades by some of the big six to billing systems have resulted in some teething problems, but the industry must do more to tackle these issues and reduce errors.”

Lyon urged customers to always check their bills carefully, and to 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,” he added.

In July, Citizens Advice said that complaints npower and Scottish Power had doubled between January and March with complaints about npower running at four times the industry average.

Scottish Power said it had recently brought in an additional 450 customer service advisers to deal with problems.

“We have transferred all of our accounts onto a new £200m customer service system and this has been challenging. We sincerely apologise to our customers who have experienced difficulties. We will ensure all issues are addressed and no customers who have experienced problems will be left out of pocket … we are continuing to add to our customer service team.”