Britain's biggest energy companies will return £153m owed to over three million UK households, as customers changed supplier or moved house.
A campaign alerting customers to refunds, which average at around £50 a piece, has been launched by the so-called "Big Six" suppliers: E.On, SSE, Scottish Power, npower, EDF and British Gas owner Centrica.
Energy regulator Ofgem ordered the firms to reimburse customers and said that if they dragged their heels enforcement action would follow.
The unclaimed money has accumulated over six years as people were unaware they were entitled to a refund when they switched to a rival firm.
The campaign is to encourage customers who have switched suppliers or moved home without submitting a final meter reading or without leaving a forwarding address, to get in touch with their old company.
"It is good news for consumers and if you think you could be owed money we recommend that you contact your previous supplier," said Dermot Nolan, chief executive of Ofgem.
"This issue is part of a wider challenge of delivering good customer service that the industry must crack if they are to rebuild customer trust and confidence.
"And while Ofgem welcomes the progress made to date, suppliers must now do everything within their powers to return the money and prevent a similar situation from happening again. Failure to deliver on the initiatives announced today could trigger further action by Ofgem, including enforcement."
Ofgem said money that remains unclaimed after two years will be used to help vulnerable customers.