Citizens Advice is warning energy customers to look past the lowest tariffs after the failure of 13 firms since November.
They have said that energy suppliers showed a "big gap" in customer service standards in their latest suppliers rankings and warned that this was the first thing to suffer when a firm is struggling.
Nabuh Energy, Utillia and Toto made up the bottom three in their latest ranking of energy suppliers, with Breeze, SSE and Igloo at the top, reports the Mirror.
Eversmart Energy would have come third from the bottom if it had not ceased trading earlier this month.
Among the common problems faced by customers of suppliers at the bottom of the rankings were billing issues such as incorrect and late final bills, aggressive debt collection practices, problems with prepayment meters such as credit refunds and faulty meters.
Citizens Advice is urging people to shop around carefully when looking for an energy supplier and recommends comparing companies' customer service performance as well as prices.
Thirteen domestic energy supply companies have gone out of business since November 2018, with six suppliers failing so far this year, affecting more than 300,000 people.
Citizens Advice chief executive Gillian Guy said: "There is still a big gap between those firms who provide an excellent service to consumers, and those who are letting people down.
"Attention-grabbing low tariffs can be a good deal, but we recommend looking deeper than just the headline price when shopping around for the best energy deals.
"We've seen six energy suppliers go bust this year. In our experience, when a company is struggling, customer service is often the first thing to suffer."
