The number of complaints made about energy giants npower and Scottish Power has doubled in three months.
Figures published by charity Citizens Advice reveal the average number of complaints per 100,000 customers of the Big Six firms is 151.
But, at npower, the figure is four times that with one in every 169 customers having complained in the three months to the end of March.
At Scottish Power, one in 506 customers complained as both firms had problems with new billing systems. Late bills often ran into the hundreds or thousands of pounds.
Npower is the subject of an Ofgem investigation but has agreed to write off outstanding payments for vulnerable customers who have not had a bill for six months. Customers who have received a late bill can contact npower on 0800 9759065.