Complaints about energy firms soared in the past year, the Energy Ombudsman has revealed.
Customers complaints jumped by 23% to 65,168 in 2015, compared to 52,937 in 2014, according to the ombudsman.
The most common problem was billing, making up 83% of grievances. Late and inaccurate domestic bills were perennial issues.
Customers who had problems switching either suppliers or tariffs accounted for 9% of complaints.
Annual breakdowns of complaints by firms will be made available by the ombudsman next month. In the third quarter of last year, ScottishPower received the most complaints, totalling 4,554, up 27% from the previous quarter. Npower came second with complaints rising by 24% to 3,581, while British Gas was third with 1,305 complaints, a 1% fall on the second quarter of last year.
Chief ombudsman Lewis Shand Smith said: "Energy complaints rose by nearly a quarter over the course of last year as customers continue to be more vocal about their discontent with suppliers."
Complaints fell to 10,896 in the final three months of the year, compared to 15,370 in the same period of 2014.
The chief ombudsman added: "Towards the end of the year, we've seen some suppliers take some encouraging steps, particularly when it comes to improving their billing processes, but there's still more that can be done.
"We encourage anyone experiencing an ongoing issue to speak to us - be it for advice or resolution, we're here to help and our complaints process is easy to use and free to consumers."