Small energy supplier Iresa has fallen to the bottom of a customer service ranking amid record complaint levels.
The company received the worst complaints score ever recorded by Citizens Advice at more than 9,000 per 100,000 customers - five times higher than the next worst supplier Toto Energy, which received 1,800 complaints.
Iresa scored just 0.35 stars out of a possible five for its customer service between January and March this year.
So Energy came top of the table with a score of 4.8 stars, while SSE was the best performer of the "big six" with 4.25.
One Iresa customer contacted Citizens Advice after receiving a request for a one-off payment of £1,000, while another person was unable to contact Iresa to get their problem resolved for more than three months.
In March, Ofgem banned Iresa from taking on new customers until it resolved a series of customer service problems.
The regulator ordered Iresa to extend its call centre hours, respond to customer emails within five working days, clear a backlog of emails, log all customer problems and offer to put all of its vulnerable customers on the priority services register.
Citizens Advice said the poor performance of some smaller energy companies and newer entrants to the market demonstrated the need for Ofgem to tighten its licensing rules.
The charity said it was currently too easy for firms to set themselves up as a new energy supplier, and its figures suggested that some suppliers entered the market before they were prepared to deliver quality customer service.
Citizens Advice chief executive Gillian Guy said: "Iresa's low score will come as no surprise for its customers, who have had to put up with continued problems. These include inaccurate billing, long phone queues and even being blocked from switching to a new supplier.
"It's vital now that Ofgem does more to protect consumers from poor customer service. The regulator's upcoming review of licensing rules must do two things: make it easier to stop unprepared suppliers from entering the market, and take poorly performing companies out of the market faster."
Ofgem interim executive director Rob Salter-Church said the regulator would "step in and take strong action" against suppliers who failed to provide good customer service.
He said: "We have already imposed a provisional order on Iresa and have warned them that its licence could be revoked unless it puts its house in order.
"We recently began a review of our approach to licensing suppliers to ensure that appropriate protections are in place against poor customer service and financial instability and will update on progress in late summer."
Energy suppliers and their Citizens Advice star rating for January to March are:
So Energy 4.8
E (Gas and Electricity) 4.3
British Gas 4.1
Bristol Energy 4.05
Green Network Energy 4.05
Bulb Energy 3.85
EDF Energy 3.85
Tonik Energy 3.85
Octopus Energy 3.8
Ovo Energy 3.55
Utility Warehouse 3.45
First Utility 3.4
Scottish Power 3.3
Co-operative Energy 3.2
Flow Energy 3.1
PFP Energy 2.85
Good Energy 2.55
Economy Energy 2.3
Extra Energy 2.3
Green Star Energy 2.2
Spark Energy 2
Avro Energy 1.7
Robin Hood Energy 1.7
Toto Energy 1.6