From today all energy firms are banned from back-billing customers beyond 12 months if the delay is not their fault - here's what you need to know.

Under new rules introduced by energy regulator Ofgem no firm will be able to bill customers for energy used more than 12 months ago if it's not the customer's fault they haven't been billed correctly.

The big six energy firms, including British Gas, E.on and EDF, had previously agreed not to bill customers beyond 12 months as part of a voluntary scheme, but Ofgem said these companies did not always follow these rules and so decided to put in place a ban that now applies to the whole sector.

Back-billing usually happens when suppliers estimate bills as they don't have regular, accurate meter readings. Once they have the meter readings they may then send a catch-up bill if the estimated bill is too low.

The regulator said the typical back-bill is £1,160 but in extreme cases it has seen customers suddenly asked to pay more than £10,000.

What are the new rules?

No firm will be able to bill you for energy used more than 12 months ago if it's not your fault you've not paid the right amount. This includes if the firm has used estimates to bill you rather than actual meter readings and there's a discrepancy.

Scenarios which could be deemed to be your fault include if you've prevented the company from taking accurate meter readings or have been stealing energy.

Ofgem says energy companies will decide on a case-by-case basis if the back-billing ban applies.

What to do if you think you've been wrongly billed

In the first instance you need to complain to your energy firm. The free Resolver* complaints tool can help draft and track your complaint.

If the problem remains unresolved, or it has been more than eight weeks since you first complained you can escalate your complaint to Ombudsman Services.