Scottish Power has received the highest number of complaints ever recorded for a UK energy company in a three-month period, according to the latest figures from Citizens Advice.

According to the charity, there were 1,163 complaints about Scottish Power per 100,000 customers in the last three months of 2014 – the highest figure ever recorded against a single supplier – and a new billing system may have been the chief culprit.

Complaints about the big six provider increased almost sixfold over the year, figures show. See's Is your energy company's service any good? poll to rate your provider's service over the past six months.

Scottish Power was banned from proactively selling its services for 12 days in March after failing to meet customer service targets set by Ofgem (see the Scottish Power banned from sales activity MSE News story for more information).

Citizens Advice says its latest figures suggest complaints about Scottish Power were the result of its new billing system, which left some people not receiving a bill and others struggling to have their problems resolved.

It adds that it's worked with Scottish Power to help resolve the problems, and the company says it has taken steps to deal with its backlog of complaints such as hiring new staff, extending call centre hours and setting up a special phone line for vulnerable customers.

Complaints about other providers down

Of the big six energy providers, SSE kept the top spot as the least complained-about, while the figures also show that the number of complaints about SSE, British Gas, EDF Energy and Npower have decreased.

Npower had been the most complained-about big six company for the first six months of 2014, but it was overtaken in the final half of the year by Scottish Power. Despite this, it still received 599 complaints per 100,000 customers in the last quarter of 2014, making it the second most complained-about company.

If you're unhappy with your supplier, complain. If you don't get a response within eight weeks or are unhappy with the reply you do get, check if Ombudsman Services will look into your complaints and take your gripe to the provider.

Alternatively, vote with your feet by joining's free Cheap Energy Club to see if you can switch and save.

'New billing systems have caused chaos for customers'

Citizens Advice chief executive Gillian Guy says: "New billing systems are routinely failing energy customers. In the last few years, four of the largest firms have introduced new billing systems, and their implementation has caused chaos for consumers.

"Thousands of customers have been hit by delayed and incorrect bills which have resulted in extreme frustration and significant debts. It's encouraging that Npower is now turning the corner, but Scottish Power still has a very long way to go."

'We apologise unreservedly'

Neil Clitheroe, chief executive of Scottish Power retail and generation, says: "We apologise unreservedly to any customers who have experienced account issues.

"These statistics reflect service problems in the last six months of 2014, when we had our most challenging period following the introduction of a new £200 million customer IT system.

"To make improvements we have added 700 new customer service advisers and we worked closely with Ofgem on call waiting times, outstanding bills and ombudsman complaints."