The gap between the best and worst performing energy firms is the widest ever, Citizens Advice figures show.
Small energy firm Extra Energy attracted 80 times more complaints than the best performing supplier SSE between April and June.
Extra Energy received 1,791 complaints per 100,000 customers, which was worse than its record low of 1,682 complaints in the previous quarter.
SSE improved its ratio to just over 22 complaints, the charity found.
Gillian Guy, chief executive of Citizens Advice, said: “The latest league table shows some suppliers are getting much better at sorting out their customers’ problems, but it’s disappointing to see others getting worse at dealing with complaints.”
The second and third worst suppliers on the list, Co-Operative Energy and Scottish Power, also received more complaints than in the previous three-month period.
Co-Operative Energy had a ratio of 850 complaints, up from 819, and Scottish Power had more than 571, up from 558.
‘Work to do’
EDF, British Gas and E.On were among the best-performing firms.
Claire Osborne, energy expert at uSwitch, said: “It’s encouraging to see some suppliers stepping up their efforts to improve their processes, but it’s clear that others still have some work to do if trust in the energy industry is to be restored.
“Inaccurate bills, one of the main causes for complaints, are unfortunately much more common than we would like.”
Extra Energy, which launched in 2014, apologised to customers and said it was now dealing with more complaints by the end of the next working day.
Ben Jones, the firm’s managing director of operations, said: “These figures reflect historic customer service issues that occurred during a period of time where Extra Energy saw our number of customers expand rapidly and unfortunately some of these complaints have taken longer than expected to resolve.
“We apologise unreservedly to anyone who has not received the standard of service we expect of ourselves.”