Energy firms have been urged to improve their complaint handling procedures after a survey showed one in four customers are unhappy with them.
Research commissioned by regulator Ofgem (Office of the Gas and Electricity Markets) found consumer gripes included the number of times they had to contact the supplier, the attitude of staff and a failure to return calls.
There were "low levels" of satisfaction across all suppliers - with billing and prices the most common grievance
- but some performed better than others, Ofgem said.
Scottish and Southern Energy and E.ON rated joint highest with 29% of customers satisfied, while nPower was lowest with 16%.
Scottish Power recorded 24%, while Centrica and EDF each polled 20% in the survey of 2,700 customers.
In a letter sent to bosses of each of the major energy companies, Ofgem chief executive Alistair Buchanan said: "It is in suppliers' best interests to ensure that the service they provide is of a high standard.
"This is clearly an opportunity for them to raise the bar to retain existing customers and attract new ones.
"With the systems and processes in place, the challenge now for companies was to really listen to what their customers were saying and look at how they could address their concerns."
Ofgem said it was investigating "concerns" that EDF had failed to comply with regulations requiring suppliers to "properly record" complaints.
An EDF Energy spokesman said: "EDF Energy takes complaints extremely seriously. The new complaint-handling arrangements were put in place less than a year ago.
"During that time, we have worked very closely with Ofgem and consumer organisations to ensure that the process works."
Garry Felgate, chief executive of the Energy Retail Association, said suppliers were working to conform with regulations governing the handling of customer complaints.
"We recognise that improvements can always be made and we look forward to working with Ofgem and consumer bodies to ensure any gaps are filled as quickly as possible," he said.