The Big Six energy suppliers are holding at least £202 million from around 3.5 million former domestic customers and £204 million from 300,000 business accounts, Ofgem has found.
The energy regulator's advice to anyone who believes they may be owed part of the money from a previous supplier is simple: "Contact the company directly and ask for it back."
The customer service phone numbers for the Big Six energy suppliers are:
If you switched away to another company or changed tenancy, Ofgem urges you to get in touch with your former supplier, preferably with a copy of the previous bill.
If you think a deceased relative may have been an account holder and was pursuing a closed account, you will need to show you are entitled to any proceeds of the estate.
Those planning on switching energy suppliers are urged to take a meter reading before the change and supply the company you are leaving with a forwarding address to ensure you receive all funds owed to you.
A spokesman for Energy UK, the trade association for the energy industry, endorsed Ofgem's advice, saying: "If you think you may be owed money, get in touch with your supplier past and present, to check if they have funds to return to you."