More than five million complained to Big Six energy firms last year
Customer complaints ranged from issues with bills, metering, customer service, switching through to payment, according to the report from Which?
Npower received 1,383,650 complaints, the most out of all six companies.
It was followed by EDF with 1,240,005, British Gas with 1,235,550 and Eon with 929,230.
Both SSE and Scottish Power received fewer total complaints for 2013 with 482,582 and 308,648 complaints respectively.
The Big Six companies account for 97% of the energy market.
Which? said the complaints figures are another indication that the energy market is not working for its customers.
The consumer group is calling on people to support its campaign that calls on Government, the regulator, competition authorities and the energy companies to drive forward radical reforms to fix the broken energy market.
Which? executive director, Richard Lloyd, said: “Millions of people are unhappy with the service they receive from the suppliers which, 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.”