The company was forced to pay back the money to about 100,000 customers who use its automated telephone meter reading system.

Customers using system between October 2003 and May 2010 have been hit by the mistake.

The fault happened when customers received an estimated bill and then used our automated EDF’s telephone system.

If they gave a higher reading, that reading and all of the additional units, for the billing period were charged at the new tariff rate.

At the same time customers who gave their readings directly to a person at EDF, were correctly billed.

An EDF spokesman said: “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.

“It is important to stress our systems have now been changed meaning customers now leaving meter readings via our automated telephone system will not be negatively impacted.”

Customers who are still with EDF will have their accounts credited automatically those no longer with the company, and who lost out by £3 or more, will receive a letter about compensation.

Luke Walsh

Action inspires action. Stay ahead of the curve with sustainability and energy newsletters from edie