More than half of EDF's customers are unhappy with the service they get when they complain to the energy giant, according to a survey by the industry regulator.
Some 51% of EDF's customers told Ofgem they were dissatisfied with their provider's procedures, the worst score out of the big six power companies.
Npower was criticised by 50% of its customers, with the remainder either being satisfied or indifferent about how their complaints were handled.
Scottish Power and Scottish & Southern Energy recorded the lowest dissatisfaction scores, of 40% and 43% respectively.
The report concluded overall satisfaction has improved since a similar survey in 2010. But Ofgem said not calling people back when agreed was an area requiring "critical improvement" by major energy firms.
Almost half of people (48%) were left dissatisfied at the speed by which their complaint was resolved, with more than a third (35%) unhappy at not being called back as promised.
Expectations not met
The Ofgem study also found a high proportion of customers did not receive confirmation a complaint was resolved.
"Expectations from customers are not particularly high or demanding of energy suppliers, but in most cases these expectations are not being met in terms of communications relating to the resolution of their complaint," the report concluded.
Audrey Gallacher, director of energy at lobby group Consumer Focus, says: "The energy industry routinely tops the charts as the sector least trusted by consumers and the customer service they receive is a significant factor in this.
"It is unacceptable that many customers have to go through a long, drawn-out process to get their problem resolved. There must be clear progress on this from suppliers.
"Ofgem has outlined moves needed by suppliers and the regulator must now ensure these improvements are delivered."
Just over 3,000 customers who had made a complaint in December last year were interviewed for the research.