The firms owe around £150m to customers. The sum has built up after the energy giants held on to the credit balances of customers who switched to a cheaper supplier.

In February, regulator Ofgem called on the energy groups to repay the balances to customers. This new scheme will allow customers to claim their money back.

Jeremy Cryer, energy spokesperson at Gocompare.com, said: “It’s a refreshing change to hear that the major energy companies will be doing more to reunite former customers with their money. Let’s hope they’ll be equally altruistic to their existing customers and pass on the benefit of substantially lower wholesale energy prices by cutting tariffs before winter hits.

“We would urge any customers who have switched their energy supplier in the last few years to go to the new myenergycredit.com website to see if there’s any credit to claim. As many as 1 in 10 ex-customers may have left a credit balance behind when they moved away from their old supplier, and although it’s great that the vulnerable and fuel poor will continue to benefit from any remaining unclaimed credit balances, I’m sure most of those who paid it in the first place would like it back.

“The major energy companies still suffer from a very poor public image and this is a step in the right direction to rebuilding consumer trust. However, this will be seen as paying lip-service to looking after customers if bills go up again in the Autumn.”

Tom Lyon, energy expert at uSwitch.com, says: “Today’s announcement is welcome news and evidence that the big six are at last beginning to put consumers first. This money belongs to their customers and it’s only right that suppliers take steps to return it. The big six have also pledged that unclaimed balances that can’t be returned will be used to benefit vulnerable customers – meaning those who are most in need of help receive it.”