Npower is the most complained-about energy firm in Britain after receiving triple the amount of complaints than its closest rival in the second quarter of 2013.
"We've had some challenges with a new computer system and we're sorry that this has caused problems for customers," said Roger Hattam, director domestic retail business at npower, while also admitting that some customers "have not had the service they deserve".
"I want to reassure them that we're working very hard and making progress in ensuring that these issues are resolved as quickly as possible."
EDF, France's state-controlled energy group, came in second with 72 complaints for the same customer sample size, during the same period.
E.ON ranked third with 60, British Gas came in fourth with 55, while Scottish Power was fifth with 41.
"Energy companies have repeatedly said they want to rebuild consumer trust," said Audrey Gallacher, director of energy at Consumer Focus.
"Good customer service and complaints handling are key ingredients to achieving this and suppliers still have a long way to go."
The data comprises complaints received by Consumer Futures as well as those made to Citizens Advice and the energy ombudsman.
The latest data shows that complaints about the big energy firms have steadily climbed from October 2012.
"People are being hit with large back bills, find their bill unclear and direct debit customers are frustrated companies are holding onto their money when they're in credit," said Citizens Advice's chief executive Gillian Guy.
"Energy companies need to put customers first and be much more transparent about where they make their profits."
Price Hikes and Bonus Pledges
The Big Six account for 99% of the UK's energy sector.
While Npower said it will raise household charges for electricity and gas by 9.3% and 11.1% respectively, Centrica said it will raise its household charges for electricity and gas by an average of 9.2% from November.
Meanwhile, its subsidiary British Gas said its electricity and gas prices will rise by 10.4% and 8.4% respectively, from 23 November.
SSE also announced it will raise its charges for electricity and gas by an average of 8.2%.
However, Ofgem data has revealed that wholesale energy prices have only risen by 1.7% despite the big six energy companies in Britain blaming this for hiking up household bills by 11.1%.
Meanwhile, Npower's chief executive Paul Massara refused to give up his bonus in light of the public outcry at the soaring cost of energy bills.
Centrica chief Sam Laidlaw's recent pledge to forgo his annual £2.6m bonus from a total pay package for 2012 that stood at £4.96m (€5.86m, $7.92m) was labelled a "gimmick" by Massara.
Massara added that he would only receive his £150,000 bonus if he hit targets tied to employee and consumer satisfaction.