An estimated three million households are being told to reclaim money they are owed by the big six energy suppliers.
The firms have launched a campaign to hand the money back, after being told to do so by the regulator, Ofgem.
On average, customers are owed around £50 each. Typically that money got left behind when they switched supplier.
Ofgem warned the companies that unless they make good progress with refunds, they will face enforcement action.
Over the last six years the large energy firms have accumulated £153m, which remains to be claimed by customers.
Often that is because customers have changed supplier, or moved home, and not left a forwarding address.
If the money is unclaimed after two years, it will be used to help vulnerable customers.
However, customers have no limit on when they need to claim by. Any valid claim will always be refunded, however old it may be.
Anyone who thinks they may be owed money is advised to visit the website www.myenergycredit.com.
"This money has been left behind, and we are urging former customers to come forward and make a claim," said Angela Knight, the chief executive of Energy UK, which represents the suppliers.
But the regulator warned the firms that they not only have to make progress in getting the money returned, but they must stop such build-ups in the future.
"Suppliers must now do everything within their powers to return the money and prevent a similar situation from happening again," said Dermot Nolan, the chief executive of Ofgem.
"Failure to deliver on the initiatives announced today could trigger further action by Ofgem, including enforcement."
He said progress was necessary if the big six were to rebuild customers' trust and confidence.
The big six are: British Gas, EDF, SSE, Scottish and Southern, E.on and Npower.
[Source: http://www.bbc.co.uk/news/business-29140613 ]
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