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:
British Gas on 0800 048 0202
EDF Energy on 0800 096 9000
Eon on 0333 202 4856
Npower on 0800 073 3000
Scottish Power on 0800 027 0072 (from landlines)
SSE on 0800 975 1662
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."