Many consumers are unhappy with the customer service they receive from gas and electricity suppliers, according to industry regulator Ofgem.
A report by Ofgem said that failing to call customers back at the specified time was the biggest complaint.
Although the report noted that overall satisfaction levels have improved since a 2010 survey, it concluded that some "critical improvements" are still required.
EDF worse for customer satisfaction
A ranking of UK energy firms found that Scottish Power and SSE retained their positions at the top of customer satisfaction standings for dealing with complaints.
In contrast, EDF Energy scored the lowest mark out of the UK's big six gas and electricity suppliers.
The consumer survey found that close to half - 48 per cent - of customers were left dissatisfied at the speed of response to their complaint.
Energy firms fail to call back
Meanwhile, 35 per cent were unhappy at the failure by energy companies to call back at an agreed time.
The Ofgem report concluded that customer expectations are not particularly taxing on energy suppliers but in many cases they are not met.
Around 3,000 customers who had made a complaint in December last year were interviewed for the research.
'Long way to go'
Audrey Gallacher, director of energy at consumer champion Consumer Focus, said: "It’s positive that there has been some improvement in how customers’ energy complaints are handled.
"However there is still a very long way to go.
She added: "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.
"Companies must remember that many people view how a firm handles their complaint as a sign of how valued as they are as a customer – so it is essential to deliver on this key issue."