The number of complaints about npower and Scottish Power doubled in the first quarter of 2014, according to Citizens Advice.
Citizens Advice and Citizens Advice Scotland point to problems with new billing systems as to why there was a sharp rise in complaints.
Complaints about npower rose from 306.8 for every 100,000 customers during the closing months of 2013, to 592.4 between January and March - or one complaint for every 168.8 customers.
Scottish Power complaints increased from 100.5 per 100,000 customers to 197.7 in the first quarter.
All suppliers saw an increase in complaints, although the rise was only slight for some, it added.
More top news
The grandson of Auschwitz's former commandant has told ITV News he could never understand how the family man could be "a mass murderer."
There are complex and contradictory ideas swirling through Germany as it digests the election in Greece. It’s a complex story.
Labour's demands for tougher safeguards on shale gas extraction were accepted by MPs but attempts to introduce a moratorium were defeated.