More than one million prepayment gas customers could have been overcharged, Energy UK has confessed.
The industry body admitted up to 1.5 million gas meters have not been working properly since they were installed, however, energy companies are contacting customers to give them a refund.
The amount will vary but most customers would have been overcharged by around 25 pence for each top-up, it said.
Angela Knight, Chief Executive of Energy UK said suppliers and the wider industry are “all very sorry” but are already getting in touch with customers.
She added: “Suppliers are working hard to roll out a solution for an issue which has affected some pre-payment gas meters. The meters were manufactured with a problem where they over-collected from customers. However, the companies know who is affected and will be getting in touch directly to put the meters right and refund any money owed.”
Ofgem said suppliers must act quickly to resolve the problem “in a way that treats customers fairly”.
Rachel Fletcher, Senior Partner for Markets added: “By the end of this week, we expect industry to: come forward with a swift timetable for resolving all traceable cases of over-charging customers, repay consumers within a clearly agreed and publicly announced timetable, reconfigure meters as quickly as possible and protect customers from over-payment in the meantime.
“We have not ruled out enforcement action.”