By Sarah Tawton
Consumer group Which? has demanded an inquiry into competition in the energy market, saying the current system is "not working for customers".
It comes after figures revealed that the Big Six energy companies received more than 5.5 million complaints last year.
Problems ranged from issues with bills and metering to customer service, switching and payments.
Which? said the figures showed that the energy market is "broken".
It called on the Office of Fair Trading and Ofgem to refer the market to the Competition and Markets Authority (CMA).
Improving trust and fair pricing
They want the CMA to investigate whether competition is working for consumers and look at ways to improve trust, transparency and fair pricing.
The first full year of official figures from energy companies shows that Npower received the most complaints in 2013, with 1,383,650.
This was followed by EDF with 1,240,005, British Gas with 1,235,550 and E.ON with 929,230. SSE received 482,582 complaints, while Scottish Power received 308,648.
These six companies currently make up 97% of the energy market.
The figures show that complaints rose to their highest level in the fourth quarter, when 1,492,065 were received.
80% of customers worried about rising bills
According to figures released by Which?, 80% of consumers are worried about rising energy prices and just 20% trust gas and electricity suppliers.
The watchdog is calling for energy companies to separate their supply and generation businesses.
And it wants firms to introduce simple pricing and swifter switching to help consumers find the best deal.
It wants the government to control the costs that are added to consumers' bills, and says energy companies must make sure charges for not paying by direct debit are cost reflective.
Which? said it wanted to see an industry "that works for customers as well as shareholders, where prices are kept as low as possible and people can trust companies to give them a good deal".
'Millions are unhappy'
Executive director Richard Lloyd said: "Millions of people are unhappy with the service they receive from the suppliers.
"Combined with low levels of trust, is yet more evidence that more must be done to fix the broken energy market.
"Next month we want the regulators to refer the energy market to the Competition and Markets Authority and launch a full scale inquiry.
"This is the first and most important step towards a more radical reform of the energy market, giving hard-pressed consumers the confidence that they are paying a fair price."