Npower has topped a list of most complained about firms, with a 25% rise in the number of unhappy customers.
Between July and September, 253.1 complaints about Npower were made for every 100,000 customers - around eight times more than the best-performing Big Six energy supplier, according to Consumer Futures.
SSE was the least complained about energy company, with 32.3 complaints per 100,000 customers, followed by Scottish Power (46.2), British Gas (55), E.ON (61.8) and EDF Energy (74.2).
Audrey Gallacher, director of energy at Consumer Futures, said: "It's very disappointing to see that since our last reporting period (April to June 2013), complaints about Npower have increased from 202.5 to 253.1 - a rise of 25%, keeping it at bottom place."
She added: "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."
In December, Npower was forced to apologise to regulator Ofgem for poor performance and admitted problems setting up customer accounts, direct debit payments and billing were affecting customers.
It promised customers would not lose money as a direct result of its billing system problems, made a £1 million payment to vulnerable customers and put a 'recovery plan' for improvement in place.
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"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," said Gallacher.
Gillian Guy, chief executive of Citizens Advice, which helped with the number crunching, added: "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."
Roger Hattam, Npower's director of domestic retail business, 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."