As many as 2.1 million households have been hit by large late bills in the last year because energy suppliers initially undercharged them, according to estimates from Citizens Advice.
The charity made the estimate after its research among more than 2,000 people across Britain found that 10% had been back-billed where energy firms send a revised bill after undercharging.
The average 'catch up' bill was for £206 but one in six people (15%) who had been back-billed said they were charged more than £250.
Citizens Advice has also seen cases where people have been back-billed for four-figure sums.
For more on dealing with energy bill problems, see our Energy Complaints guide.
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What is back-billing?
Back-billing happens when customers have been undercharged for their energy for a period of time.
It can be because a customer did not pay a bill, but it can also be due to suppliers underestimating bills or not investigating a technical fault, Citizens Advice says.
Suppliers can back-bill customers for up to 12 months' worth of gas or electricity, even when it was the firms' fault. Customers can be back-billed for longer periods when the supplier argues the customer was at fault.
What can you do?
Hit with a large late bill? We've full help in the Energy Complaints guide, but in brief:
- Firstly, contact your supplier to try to resolve the problem. If the billing issue was for 12 months or longer and it was the supplier's fault, you should not have to pay the bill.
- If this is not resolved to your satisfaction within eight weeks, you should go to the free Energy Ombudsman.
- Alternatively you can use the free online tool Resolver*, which we work with it'll help draft your complaint, manage it and escalate it to Ombudsman Services too if needed.
'Disappointing' from energy firms
Energy secretary Amber Rudd says: "This is very disappointing. I challenged the UK's biggest energy suppliers to bring down bills and pass on savings to consumers. They have dropped their prices which is a step in the right direction, but errors on bills create unnecessary burdens and worry.
"We're installing smart meters to help people take control of their energy use and bring an end to estimated bills, as part of our plan to build a system of energy infrastructure fit for the 21st century."
Gillian Guy, Citizens Advice chief executive, says: "Customers shouldn't have to pay the price for suppliers' mistakes.
"Energy bills are already high so it adds insult to injury when companies go back to customers looking for more money after they got it wrong."
Additional reporting by Ben Salisbury.