Npower is dealing with the fallout of throwing its customers' finances into "chaos" having been named most-complained about energy supplier of 2013.
The company remained at the bottom of a complaints league table compiled by Citizens Advice every quarter during the year, and had done so since the end of 2012.
It received 306 complaints for every 100,000 customers between October and December 2013 - 10 times as many as its Big Six rival SSE, which at 31 complaints per 100,000 customers had the fewest unhappy customers.
Bills arriving late, payments being stopped and new accounts not being set up were some of the issues cited by frustrated Npower customers, some of which stemmed from the company's introduction of a new billing system last year.
Chief executive of Citizens Advice, Gillian Guy, said: "For well over a year now some Npower customers have been finding their finances thrown into chaos. Some are not receiving bills and others are ending up in debt because their direct debit was cancelled. Citizens Advice has asked Npower to make sure people affected get any appropriate compensation."
A spokeswoman for the company said it had written to all its customers during the final quarter of last year apologising for the problems with the new billing system.
"We are making good progress in dealing with the root causes of this, but remain totally committed to resolving any problems this has created for our customers," she added.
How the rest of the big Six got in in Q4 2013
- SSE (31 complaints per 100,000 customers)
- British Gas (53)
- E.On (55.8)
- EDF (62)
- Scottish Power (100)