Energy firm Npower has topped a list of complaints compiled by a leading consumer watchdog, with its performance branded as "unacceptable".
The data, released by Consumer Futures, shows Npower notched up 202 complaints per 100,000 customers between April and June 2013 - five times more than the best-performer, SSE.
The figures were compiled before the latest round of energy price rises, indicating the rate of complaints received by the Big Six firms could soon rocket.
The data shows that EDF Energy came in second for the most complained about Big Six energy firm, recording 75 complaints per 100,000 customers, followed by E.On (60 complaints per 100,000 customers), British Gas (55), Scottish Power (41), and SSE (38).
Roger Hattam, director of domestic retail business at Npower, said: "We know that some of our customers have not had the service they deserve. We've had some challenges with a new computer system and we are sorry that this has caused problems for customers.
"I want to reassure them that we are working very hard and making progress in ensuring that these issues are resolved as quickly as possible."
A long way to go
Of all the firms announcing winter price rises, Npower's increase was by far the worst - it announced an average rise in bills of 10.4%.
British Gas customers will have to stump up 9.2% more, while Scottish Power customers will pay 8.6% more and SSE customers will see their bills rise by 8.2%. EDF Energy has also announced a 3.9% increase, while E.On is expected to usher in a rise of around 6.6%.
Audrey Gallacher, director of energy at Consumer Futures, said the Big Six energy companies had a "long way to go" before they could restore trust with British consumers, adding that Npower's performance is "unacceptable and the company must take further steps to tackle this.
She said: "Energy companies have repeatedly said they want to rebuild consumer trust. Companies must remember that many people view how a firm handles their complaint as a sign of how valued they are as a customer, so it is essential to deliver on this key issue."