An energy firm has apologised and reimbursed nearly £200,000 to tens of thousands of customers after identifying a fault with its automated telephone meter reading system.
The problem affected EDF Energy customers and meant some were overcharged when the company changed its prices between October 2003 to May 2010.
The energy provider said "in the vast majority of cases the impact on customers has been minimal" and for many customers who have been overcharged the amount involved is less than £5.
It said some customers have benefited from being under-billed.
In a statement, the company said: "EDF Energy has resolved a fault with its automated telephone meter reading system. The problem only affected customers who used this system during periods when EDF Energy changed its prices between October 2003 to May 2010.
"Naturally, we will not be looking to recover any money from those customers who have been under-billed and of course have plans in place to fully reimburse those that have been over charged, including an amount for accrued interest.
"This figure totals just under £200,000 to be reimbursed across a total of 100,000 customers. It is important to stress that our systems have now been changed, meaning that customers now leaving meter readings via our automated telephone system will not be negatively impacted.
"Following price changes between October 2003 to end of April 2010, customers who received an estimated bill and then subsequently used our automated telephone system to provide a higher reading saw all of the additional units for the billing period charged at the new tariff rate. In contrast, customers providing their readings directly to a customer adviser saw their charges correctly allocated over the full billing period resulting in an accurate bill."
The company said it had identified all of the customers who were impacted either positively or negatively and customers who are still with the company need not do anything as their accounts will be credited automatically. The company also intends to write to customers who have left, and who lost out by £3 or more, with details on how to claim the refund they are due.
The statement added: "Energy regulator Ofgem has investigated this issue and we have advised the regulator on how we intend to resolve this matter. We have identified all of the customers who were impacted either positively or negatively.
"We would like to apologise to those customers impacted by this issue and reassure them that as soon as it was identified, corrective action was taken."
Responding to the announcement, Adam Scorer, director of external affairs at Consumer Focus, said: "Customers clearly have a right to expect that if they take the time and effort to give a meter reading that their supplier will reflect this in their bill.
"Energy firms have a responsibility to ensure that they charge for the right amount of units at the old and new rates when prices change, and if they have an up-to-date meter reading there is no excuse for getting it wrong.
"It is welcome that EDF Energy are doing the right thing and will be compensating their customers for the overcharging problems that their automated meter reading caused.
"We know that some customers are concerned about whether firms are charging them incorrectly around energy price changes. So, it is also welcome that Ofgem is talking to other suppliers about their automated systems and their charging at old and new unit rates when prices change."