Scottish Power has received the highest number of complaints ever recorded for a UK energy company in a quarter, according to the latest figures from Citizens Advice.
There were 1,163 complaints about Scottish Power per 100,000 customers in the last quarter of last year, the national charity and consumer advocate said.
It is the highest figure ever recorded against a single supplier, ending a year in which the total number of complaints about the Big Six provider increased by 488%, the figures show.
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The Spanish-owned company was banned from proactive sales for 12 days in March after failing to meet customer service targets set by Ofgem.
The regulator found the firm had not made necessary improvements to respond to customer complaints.
Citizens Advice said its latest figures suggested 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.
The charity said it had worked with Scottish Power to help resolve the problems, and the company had 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.
The figures also show that the number of complaints about SSE, British Gas, EDF Energy and Npower decreased.
Npower had been the most complained-about member of the Big Six for the first two quarters of 2014 but was overtaken in the final half of the year by Scottish Power.
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However, while Npower had made improvements, it still received 599 complaints per 100,000 customers in the last quarter of last year, making it the second most complained-about company, the charity said.
Citizens Advice chief executive Gillian Guy said: "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."
Citizens Advice Scotland chief executive Margaret Lynch said: "Trust in the energy market is at rock bottom among consumers in Scotland, so it's really disappointing to see that certain suppliers are under-performing.
"We saw a staggering increase in the number of Scottish Power issues brought to the Citizens Advice service last year. The league table reveals that complaints to the supplier increased almost six fold in 2014. This highlights a truly shocking level of customer service."
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Neil Clitheroe, chief executive of Scottish Power retail and generation, said: "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. We also agreed with Ofgem to stop outbound sales for a two week period.
"Recently our call answering times have been among the best in the industry and outstanding complaints have been reduced significantly. We remain fully committed to resolving outstanding issues and ensuring that no customers will be left out of pocket."
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