By Daniel Hunter

Scottish Power received more complaints in the final three months of 2014 than have ever been recorded by an energy company.

Citizens Advice said there were 1,163 complaints made for every 100,000 Scottish Power customers. It's the highest number ever recorded against a single energy supplier as figures show complaints against the 'big six' energy firms was up 488% in 2014.

Scottish Power was banned from actively seeking new customers for 12 days after not meeting customer service targets set out by energy regulator Ofgem.

Citizens Advice said the surge in complaints towards the end of 2014 were as a result of a new billing system. Problems with the new system left customers unable to access/receive their bills and weren't able to have their issues resolved.

Gillian Guy, Citizens Advice chief executive, 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.”

Neil Clitheroe, the 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.”