Co-operative Energy has blamed ongoing problems with its new IT system after it received the highest ratio of complaints in the third quarter.
The energy supplier upgraded its computer systems in March, with many customers complaining of being locked out of their online account or not receiving their bills following the switch.
And new figures from the energy ombudsman revealed Co-operative Energy received 136 complaints per 100,000 customers in the three months to September, 50% higher than any other supplier.
Responding to the findings, a Co-operative Energy spokesperson claimed it had made "significant progress" on resolving its technical issues.
"We would like to thank customers for their patience and continued support in recent months," the spokesperson said.
"We have made significant progress to resolve a number of technical issues which ultimately related to the introduction of a new IT system. We will continue to make further improvements to ensure our customers receive the high level of service they expect and deserve."
Looking at the broader industry, the energy ombudsman said it had received a total of 13,212 complaints in the third quarter, slightly lower than the previous quarter's total of 13,490.
Disputed charges, inaccurate invoices and missing bills were by far the most common complaints, accounting for nine out of every 10 (87%) complaints received by Ombudsman Services.
Commenting on the findings, chief ombudsman Lewis Shand Smith said: "Consumers are rightly more demanding of their energy suppliers than ever before, taking service as well as cost into account.
"Having easy access to these figures gives consumers the power to make informed decisions about their energy provision."
Below is a breakdown of the total number of complaints received per 100,000 customers, according to Ombudsman Services.
Co-operative Energy - 135.92
Scottish Power - 89.20
Npower - 71.39
First Utility - 41.32
E.ON - 14.50
Ovo Energy - 10.41
British Gas - 8.94
EDF Energy - 7.33
Utility Warehouse - 5.62
SSE - 3.88