Economy Energy has been ranked the worst for customer service among suppliers for a second consecutive quarter following complaints about billing errors and its communication system.
The company's score of 1.8 out of five in the Citizens Advic
e customer service table was almost matched by Spark Energy, which dropped 11 places to 18th position with a score of 1.85 for the period from June to September.
One Economy Energy customer was forced to seek help from Citizens Advice after five months of trying to contact the company for an accurate bill.
Other Economy customers reported billing errors when they switched supplier and sought help after they were unable to contact their supplier.
Spark Energy customers also reported billing issues to Citizens Advice as well as problems with how the company collects debts.
One customer reported receiving a bill of £1,500 from Spark for a property they had moved out of.
First Utility rose to the top of the table with a score of 4.4 out of five.
The star rating system is based on a combination of five factors, including how complaints are dealt with and how highly customers rate the supplier's customer service.
Citizens Advice chief executive Gillian Guy said: "When it comes to customer service, some energy suppliers appear to be getting worse rather than better.
"It's encouraging that some companies have made improvements in how they treat their customers, but we're concerned that customers are still being let down through inaccurate billing or aggressive debt collection practices.
"Energy companies must do more to improve their offer to to consumers. This means dealing with complaints effectively, being easy to contact, and being clear in the bills they send customers."