The company received 253 complaints per 100,000 customers, eight times more than the best performing company, SSE.

The data, collated by watchdog Consumer Futures, covers the period from July to September 2013.

Complaints about npower have increased by 25% since the last reporting period between April and June 2013, keeping it at bottom place.

In December npower agreed to pay a £3.5m fine for breaching sales rules, after the regulator Ofgem found the company had broken the rules governing how deals can be sold over the phone and on the doorstep.

In a statement issued today, Roger Hattam, director of domestic retail business at npower 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. Our customers deserve to get the best service possible and this is my commitment to them.”

However, Consumer Futures said the deterioration in npower’s performance was “wholly unacceptable”.

“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 Consumer Futures’ director of energy Audrey Gallacher.

She added that energy companies have “repeatedly said they want to rebuild consumer trust” but “still have a long way to go”.

The report found that EDF Energy received the next highest number of complaints (74 per 100,000 customers) followed by E.ON (62 per 100,000 customers), British Gas (55 per 100,000 customers) and Scottish Power (46 per 100,000 customers).

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.”