The energy giant has alerted the regulator, Ofgem, after it discovered 350 existing gas customers were incorrectly charged. The firm said all overcharged amounts will be refunded and customers will receive interest and ‘appropriate compensation’.

The issue was down to a mismatch of the metric and imperial units so customers with a metric meter had been registered as having an imperial meter or vice versa.

While the cause of the issue has yet to be fully determined, Ofgem believes human error and poor data flows between meter installation, exchange and change of suppliers, were the reasons behind the problem.

It meant the conversion factor of 2.83 between cubic feet and cubic metres had been incorrectly applied or not applied at all.

E.ON confirmed customers who were undercharged won’t have to pay any money back and the firm will investigate whether former customers are due compensation.

But the discovery throws up “a potentially industry wide meter set-up issue”, E.ON said.

As a result, Ofgem has written to all gas suppliers to let them know they too could be impacted by the same blunder.

Suppliers have been told they are responsible for identifying affected customers and providing redress.

The letter to suppliers’ states: “Upon determining the full extent and impact of the issue, Ofgem will work with affected suppliers to agree a timetable for putting things right, including resolving all traceable cases of mischarged customers and preventing this issue from further impacting other customers.”

A statement by E.ON said: “Within the customers it currently supplies, E.ON has identified all those affected by this issue where the affected meter is still in situ and continue to work to identify any historic meters that have subsequently been exchanged that may have been impacted. E.ON will contact all customers individually. Customers do not need to take any action.”