If you've switched energy provider in the last six years, you could be owed a slice of £200 million after energy regulator Ofgem found the big six suppliers guilty of failing to return money to millions of former customers.

Ofgem says British Gas, EDF, E.on, Npower, Scottish Power and Scottish and Southern Energy (SSE) are holding onto at least £202 million from around 3.5 million former customers, who overpaid on their bills before switching elsewhere (join our free Cheap Energy Club to see if you can save by switching).

In addition, 300,000 business customers are owed £204 million.

The regulator says these amounts are "unacceptably large" and "expects suppliers to do more" to return the money.

It adds suppliers must also give "crystal-clear communication" to consumers about what to do when closing an account to ensure they don't lose out.

Where it isn't possible to repay a balance, suppliers should find ways to use the money to benefit consumers – such as donating it to the Government's Warm Home Discount scheme.

The news comes after five of the big six changed their policies this month so more customers will have overpayments returned to them (see the Millions to get energy bill credit refunds MSE News story).

Ofgem says it is continuing to look at whether suppliers' current policies and practices comply with its rules, including those which tell firms to treat customers fairly.

'Don't take it lying down'

MoneySavingExpert.com energy analyst Archna Luthra says: "This is yet another way misbehaving energy firms are punishing consumers – it's outrageous they have got away with holding onto cash that doesn't belong to them for this long.

"Don't take it lying down – if you've switched in the last six years, contact your old supplier and demand your cash back.  

"Ofgem needs to crack down on this even harder and force firms to automatically refund credits when people switch – or risk undermining the whole switching process in an already apathetic market."

I think I'm owed money. What can I do?

If you've switched to another energy supplier or moved address in the last six years, Ofgem says you should get in touch with your former supplier, preferably with a previous bill handy, to check whether or not you're owed any money back.

This process applies to all customers who were in credit when they switched, except those on prepayment meters.

If you switched supplier or moved address longer than six years ago, you can still try to claim your money back by speaking to your then supplier. But it will be difficult if you haven't kept records and old bills.

If you think a deceased relative may have been an account holder and are pursuing a closed account, you need to prove you're entitled to the proceeds of their estate.

If you're about to move home, make sure you take a final meter reading and give your new address to the company you're leaving. That way, you've made it easy for the firm to contact you and give you the refund you're due.

Additional reporting by the Press Association.