COMPLAINTS about npower and ScottishPower doubled in the first quarter of this year after new billing systems caused problems.
Citizens Advice Scotland said complaints about npower rose from 306.8 for every 100,000 customers in the last quarter of last year to 592.4 between January and March.
The charity's latest complaints league table sees npower "firmly at rock bottom" in sixth place and ScottishPower in fifth place.
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A ScottishPower spokesman said: "Over the last six months we have been installing a new £200 million account management system, which will mean real improvements to our customer service. However, the transition is difficult and has led to a dip in customer service levels."