The six largest energy suppliers in the UK have been warned by Ofgem that they must improve their customer service levels.
According to Alistair Buchanan, chief executive of the energy regulator, poor complaint-handling procedures need to be improved before the regulator looks at the issue again in a year's time.
"It is in suppliers' best interests to ensure that service they provide is of high standard," Buchanan said.
"This is clearly an opportunity for them to raise the bar," he added.
Among consumers' biggest concerns was the number of times they had to contact their energy supplier to resolve a billing issue or other problem.
Bill payers were also disappointed that promises by call centre staff to return their call were often not kept.
Meanwhile, Consumer Focus, an independent watchdog for the energy sector, claimed that electricity and gas suppliers have not been passing on the cuts in wholesale energy prices to their customers in full.
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