A fifth of UK householders complain to their energy providers every year.

A new report from charity Citizens Advice also revealed energy suppliers spend around £125 million a year to handle the complaints.

It said progress on billing is still “patchy” and “poor processes” have led to distrust among consumers.

One of the main reasons for the rise in billing problems is due to the changes in suppliers’ billing systems, which resulted in major service failures as well as delayed bills and the build-up of debts.

Gillian Guy, Chief Executive of Citizens Advice said: “Bad billing systems have plagued energy customers for years.

“Billing people on time and for the right sums of money is essential if the market is to work.  It is both astonishing and wholly unacceptable that suppliers have been getting it so wrong for so long.”

The report is calling on Ofgem to review why the pressure of competition and regulatory requirements has failed to bring down billing problems.

It is also urging the regulator to work with customers to understand what information they need and remove “unnecessary information” from bills.

Ofgem said it “shares Citizens Advice’s concerns” about poor billing practices and customer service in the energy market.

A spokesperson added: “Ofgem is taking action to hold suppliers to account for poor customer service. For example, we have introduced requirements on suppliers to treat their customers fairly and have opened two investigations into suppliers (npower and ScottishPower) because of concerns about their customer service. Ofgem is concerned the energy market is not working effectively for consumers and that is why we have asked the Competition and Markets Authority (CMA) to investigate. We will wait for their findings and any remedies before deciding in close consultation with Government and consumers bodies what further action is required to fix this market for consumers.”

ELN has contacted Energy UK for a statement.