Extra Energy has attracted the highest number of customer complaints among energy companies for a second consecutive quarter, performing 80 times worse than the best-performing supplier, figures show.
The relative newcomer attracted the highest complaints ratio ever recorded in the Citizens Advice quarterly league table, with 1791.2 per 100,000 customers, up from 1,682.0 last time.
The provider, which has more than 500,000 customers, has consistently offered some of the cheapest tariffs in the market since its launch in early 2014.
Common problems reported to Citizens Advice about Extra Energy included late or inaccurate bills and difficulty getting through to its customer service team on the phone.
The gap between how well the best and worst-performing suppliers were at dealing with customer complaints, based on the seriousness of the problem and how long it took to resolve, was the widest on record, Citizens Advice said.
SSE was the best performer with 22.5 complaints per 100,000 customers, compared to 28.4 during the previous quarter.
EDF came second with a ratio of 30.5, followed by E (35.8). British Gas took fourth position with a ratio of 79.5.
Flow Energy, the last quarter's second-best performer, dropped to tenth place with a fourfold increase in its complaints ratio.
Npower's complaints dropped by more than a third but it remained in the fourth-worst position, while Co-operative Energy and Scottish Power took second and third-worst positions with 850.3 complaints and 571.5 complaints per 100,000 customers respectively.
Utilita more than halved its ratio, while complaints at Ecotricity doubled compared with the last quarter.
Citizens Advice chief executive Gillian Guy said: "People shouldn't face a gamble on getting good customer service.
"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.
"Patchy and unpredictable customer service affects people in many ways - from the financial stress of being hit with a late bill, to the time wasted trying to get hold of a supplier on the phone."
Consumers Minister Margot James said: "Poor customer service can erode trust and cause unnecessary stress for bill payers.
"It's great to see that some companies are improving, but all energy suppliers should be treating their customers fairly and ensuring that any complaints are suitably addressed. If they don't, their customers will vote with their feet and switch to a different supplier."
Ben Jones, Extra Energy 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.
"Resolving complaints promptly is incredibly important to us and we have continued to see improvements in the level of complaints we resolve by the end of the next working day. We apologise unreservedly to anyone who has not received the standard of service we expect of ourselves."