£400m is thought to be owed due to credit left in customer accounts accumulated over the past six years from moving house or switching suppliers.
It's estimated that half of the money is owed to 3.5m domestic customers. Ofgem is expected to make an announcement on the issue on Friday.
The news comes only two days after Ofgem outlined plans for further scrutiny of the accounts of the big six energy companies, as well as attempts to remove barriers to entry for new competitors.
The regulator confirmed that a range of new measures will make the annual statements of the larger energy companies more useful and accessible.
Ofgem also announced it will review the method used for reporting the price of electricity sold by the largest suppliers’ generation businesses to their supply businesses in their annual statements.