Despite acknowledging over a year ago that its new billing system was causing problems for consumers, there has not been any improvement in complaint numbers, the consumer body said.

Bills arriving late, payments being stopped and new accounts not being set up are just some of the problems npower customers experienced with the new billing system.

Annual figures reveal npower topped the complaints leader board every quarter last year.

The company received 306 complaints for every 100,000 customers in the last quarter of 2013, meaning one in every 326 customers made a complaint.

Citizens Advice chief executive Gillian Guy said: “Things are getting worse not better for npower customers. It is unacceptable that npower has not yet sorted out the serious failings in its billing systems and customer service which are causing so many complaints and serious problems for its customers.

“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 and of Citizens Advice Scotland are calling on npower to compensate people affected.

Npower wrote to all its customers last year apologising for the problems with the new billing system.

In a statement the firm said: “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.”

Number of complaints against the Big Six suppliers: October- December 2013