New figures show npower remains the most complained-about gas and electricity supplier in the UK, with a 25 per cent rise in customer gripes.
Npower received more than 250 complaints for every 100,000 customers between July and September last year.
The firm's complaints figures represent a rise of 25 per cent on the previous three-month period.
This is according to data from watchdog Consumer Futures.
The high level of customer dissatisfaction with the company is due to problems caused by the introduction of a new billing system, said the organisation.
SSE lowest number of complaints
Rival gas and electricity supplier SSE had the lowest number of complaints at 32 per 100,000.
Among the remaining big six energy firms, 46 out of every 100,000 ScottishPower customers complained in the same period, while at British Gas the rate was 55.
The complaint rate at E.ON was 62 out of every 100,000 customers and at EDF it was 74.
Last month, the energy industry regulator Ofgem forced npower to pay £1 million in compensation to its most vulnerable customers as a result of issues caused by billing errors.
Audrey Gallacher, director of energy at Consumer Futures, called npower's complaints record "disappointing".
Npower performance 'wholly unacceptable'
"Npower has acknowledged and apologised for its poor billing systems and agreed to pay vulnerable customers who have been disadvantaged," Gallacher said.
However, she added, the firm's "deterioration in performance is still wholly unacceptable".
"The company must address the failure of its systems, processes and customer service to put things right."
Gallacher added that she expected energy regulator Ofgem to keep npower under close scrutiny to ensure that none of its customers lost out financially as a result of these problems.
Customer service important
"Energy companies have repeatedly said they want to rebuild consumer trust," she said.
"Good customer service and complaints handling are key ingredients to achieving this and suppliers still have a long way to go."
Gallacher said that consumers were interested in the kind of service they could expect to receive from a supplier, not just the price of its tariffs.
Kate Rose, head of gas and electricity at Confused.com, added: "When comparing tariffs we recommend that customers consider a number of factors alongside the potential savings.
"On our results table we clearly show each supplier's customer ratings, which are based on real customer feedback."
Npower apology for customer service 'issues'
An npower spokesman said: "Last year, we apologised to our customers for the service issues they may have faced, following the installation of a new billing system.
"We're working on these issues as top priority and, while we still have a long way to go, we're making good progress."
At the end of 2013, npower raised gas and electricity prices by an average of 10.4 per cent.
As a result of the government decision to cut environmental taxes and obligations on energy firms, the company has since announced a 2.6 per cent cut in tariffs.
But this nonetheless means that npower customers have been hit with the highest overall increase in the cost of gas and electricity this winter when compared with its big six rivals.
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