Which? Slams UK Energy Market As 'Broken'
Britain's energy market is "broken" and should be referred for a full competition inquiry, according to a consumer group.
Which? has published figures showing that the so-called big six companies received more than 5.5 million complaints last year.
It said npower was the worst offender - with almost 1.4 million complaints.
The next on the list was EDF, followed by British Gas, E.on, SSE and Scottish Power.
Which? said complaints ranged from issues with bills and metering to customer service and ease of switching to competitor firms.
It has urged for an investigation into how competition could be increased to deliver trust, transparency and fair prices.
Supplier trade group UK Energy admitted changes were needed.
In response to the report it said: "No industry wants unhappy customers but when you are dealing with nearly 27 million customers things can occasionally go wrong.
"Energy companies put a great deal of effort into resolving any problems their customers may have and most complaints are fixed within a couple of working days and with no more than a phone call.
"Energy companies are already making a number of changes which put customer service at the heart of what they do.
"They are working to make switching easier and quicker and new rules also mean the industry is cutting the number of tariffs and making energy bills clearer to understand."
Meanwhile, alternative provider Ovo Energy has announced a 2.5% bill cut, citing mild winter weather.
It said up to £180 would be saved by consumers compared to tariffs imposed by the main suppliers.
Ovo chief executive Stephen Fitzpatrick said his company also believed the energy market was dysfunctional.
He said the big companies relied on regular profit coming from consumers who did not bother switching for the best deals.
According to a statement released by price comparison service uSwitch, the best buy energy tables are now dominated by small suppliers undercutting the big firms.
Energy expert Tom Lyon said: "By making these changes, Ovo is sending out a clear signal that it intends to woo consumers from the arms of the big six.
"By dominating the best buy tables smaller suppliers are making sure that consumers will give them more than a passing glance.
"They are positioning themselves carefully knowing that once a consumer checks them out it's highly likely that their other attributes will draw them in."
:: Watch Sky News live on television, on Sky channel 501, Virgin Media channel 602, Freeview channel 82 and Freesat channel 202.