WORCESTER energy giant Npower received the greatest amount of complaints of any of the top six power suppliers in the country in a three-month period last year.
A report by watchdog Consumer Futures found the company received 253.1 complaints per 100,000 customers fromJune to September last year - about eight times as many as competitor SSE.
This is an increase of 25 per cent on the previous three months and is more than three times as many as EDF Energy, which had the next greatest number of complaints at 74.2 per 100,000 customers.
Director of energy with Consumer Futures Audrey Gallacher described the figures as "very disappointing".
"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," she said.
She called on energy regulator Ofgem to continue to closely monitor Npower and the company to work to drive the figures down.
"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.
"Along with price, good service is important to customers. People want to know the relative performance on complaint handling to help them make informed choices when deciding whether to switch."
SSE recorded only 32.3 complaints per 100,000 customers in the same period, followed by Scottish Power at 46.2, British Gas at 55, Eon at 61.8 and EDF Energy at 74.2.
Last year the company introduced a new billing system, which led to problems for a number of customers.
Npower's director of domestic retail business Roger Hattam said the company was working to increase customer satisfaction.
"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," he said.
"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."