EDF Energy has revealed that 100,000 customers have been overcharged due to a fault with its automated telephone meter reading system.

In an announcement today, the energy company said that it had resolved the fault, which affected customers who used the automated system during the periods when EDF Energy changed its prices between October 2003 to May 2010.

The company claims that most customers have only been overcharged by less than £5, but admitted that one customer was forced to overpay by £500.

“In the vast majority of cases the impact on customers has been minimal. It means some of our customers who provided their own meter reading via our automated telephone system have been overbilled, but equally some have benefited from being under-billed,” the company said.

A spokesperson for the company said that the faulty system was likely to be an in-house legacy system.

EDF said it will not chase customers who have been under-billed, but will fully reimburse customers who were overcharged, including an amount for interest. It expects to pay out just under £200,000 to 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,” the company added.

Customers who provided meter readings directly to a customer adviser during the seven-year period were not affected by the fault.

Existing EDF Energy customers who have been affected will have their accounts automatically credited by 30 September. The company will write to the address it holds of customers who have left EDF, who were overcharged by at least £3, and provide details of how they can claim a refund.

Energy regulator Ofgem investigated EDF’s case, but it will not be imposing a fine because the company did not break a specific condition of its licence.

However, Ofgem said it would be writing to other energy companies to gain assurance that they have not got similar issues with their voice recognition software.

A spokesperson for the regulator emphasised that it did not have reason to suspect that the problem was pervasive because each energy company uses different software.

Earlier this year, EDF signed a £100 million contract with Capgemini for a range of IT support services in a bid to cut desktop support costs.

Under the three-year contract, Capgemini will provide procurement, service desk and managed desktop services to the energy company's 15,000 IT users.