Energy regulator Ofgem has said it "expects suppliers to do more" to ensure that money held in credit from closed accounts is returned to former customers.
Ofgem said an inquiry into the suppliers had found "an unacceptably large amount of money being retained rather than returned to consumers".
Most of the major suppliers, who are known as the Big Six, have recently pledged to automatically refund surplus money to existing direct debit customers.
Ofgem has said it expects the companies to do all they can to return money to individual consumers and to inform current consumers clearly what to do when closing an account.
Ofgem interim chief executive Andrew Wright said:
When many people are struggling to make ends meet, it is vital that energy companies do the right thing and do all they can to return this money and restore consumer trust.
We want to see decisive action by suppliers, individually and collectively, to address this issue and, wherever possible, to ensure that the balances they currently hold are returned to consumers.