Co-operative Energy has blamed problems with a new IT system for netting the highest complaint rate ever recorded in a quarterly customer service league table.
The rankings of the 19 largest household energy providers, compiled by Citizens Advice, showed the company attracted 1,584 complaints per 100,000 customers between July and September.
The advice charity said it amounted to 40 times more than those facing "big six" firm SSE, which performed best over the period with 39 complaints per 100,000 customers.
Common themes among those taking issue with Co-op were failures over switching, billing and even being locked out of their online accounts.
Co-op Energy, part of the Midcounties Co-operative, started business in 2010 and has grown to achieve a customer base of 240,000.
But it has slid down the customer service league during the course of this year.
The company said of the Citizens Advice figures: "This data is from July, August and September 2015 when unexpected issues relating to the implementation of our new IT system were at their peak.
"Since then, we have fully resolved 93% of complaints that were referred to the Ombudsman and are working through any remaining issues.
"The level of service our customers now receive has significantly improved. Call response times have halved over the past six weeks and we have recruited and trained over 100 new customer service staff.
"We are making further improvements to ensure our customers receive the high level of service they expect and deserve."
It is not the only energy supplier to have felt some heat over new billing systems.
Npower was ordered this summer to give some customers free energy for failures in its handling of their complaints.
ScottishPower has also faced pressure over high complaint volumes in the past - coming bottom in the last Citizens Advice table - but showed improvement this quarter with 740 complaints per 100,000 customers.
Chief executive of Citizens Advice, Gillian Guy, said: "Good customer service is essential to any effective industry. The fact energy consumers face such a lottery shows this market has a long way to go.
"While it's encouraging to see some companies sharpening up their act, some firms at the bottom are getting even worse."
The charity pointed to 1,164 complaints per 100,000 customers of Extra Energy as one example.
The company has apologised and insisted its move to larger offices will help it improve.
Managing director of operations, Ben Jones, added: "Call waiting times have reduced by more than 90%. We appreciate the affected customers bearing with us while we work to provide them with the quality service that they rightly expect."