Npower has continued to receive the highest number of complaints among energy suppliers - around eight times more than the best performing company, figures show.
Consumer Futures said Npower received 253.1 complaints per 100,000 customers, placing it bottom of the big six suppliers over the three months to September.
SSE recorded the lowest number of complaints at 32.3 per 100,000 customers, followed by Scottish Power (46.2), British Gas (55), E.On (61.8) and EDF Energy (74.2).
Consumer Futures director of energy Audrey Gallacher said: "It's very disappointing to see that since our last reporting period, complaints about Npower have increased from 202.5 to 253.1 - a rise of 25%, keeping it at bottom place.
"While Npower has acknowledged and apologised for its poor billing systems and agreed to pay vulnerable customers who have been disadvantaged, its deterioration in performance is still wholly unacceptable and the company must address the failure of its systems, processes and customer service to put things right.
"We expect Ofgem to monitor Npower closely, so that any customers who have been a victim of poor billing practices will not lose out financially. And we expect Npower to do all it can to identify and rectify such cases.
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Citizens Advice chief executive Gillian Guy said: "Rising energy bills are eating into people's already stretched budgets, often consuming money that could have been spent on other daily essentials like food.
"With people paying such a high price to have a warm home or keep the lights on, the service provided by energy companies should be exemplary, but all too often we're finding that's not the case. The findings from Consumer Futures should serve as a stark reminder to energy firms that there is much more for them to do in order to improve service and increase consumer trust."
Npower's director of domestic retail business, Roger Hattam, said: "Last year, we apologised to our customers for the service issues they may have faced following the installation of a new billing system. I know that we've let many of our domestic customers down and I want to apologise personally for this and promise that they will not lose out financially as a direct result of these issues.
"We're working on these issues as top priority and, while we still have a long way to go, we're making good progress. Our customers deserve to get the best service possible and this is my commitment to them."
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