UK energy suppliers have been told to review the way they charge customers by direct debit amid complaints about soaring charges and credit balances.
The regulator Ofgem also urged six suppliers to take "immediate" action to tackle issues with their charging processes.
Some 500,000 households saw direct debit payments double this year, Ofgem said, despite prices rising by 54%.
It wants suppliers to check whether such a large increase was justified.
The six suppliers singled out included Ecotricity, Good Energy, Green Energy UK, Utilita Energy, TruEnergy and UK Energy Incubator Hub (which has now ceased trading).
Ofgem found they had "moderate to severe" weaknesses, ranging from inadequate charging processes to lacking a structured approach to setting direct debits.
The watchdog has now ordered all energy suppliers to review the accounts of customers whose direct debit was doubled or more between February and the end of April.
Where appropriate, Ofgem said it expected suppliers to adjust any miscalculations, including making repayments if needed, and consider whether a goodwill payment is warranted.
"Suppliers must do all they can, especially during the current gas crisis, to support customers and to recognise the significant worry and concern increased direct debits can cause," said Jonathan Brearley, Ofgem's chief executive.
"We know there is some excellent service out there, but we want to make sure that it's consistent and standard across the board. It's clear from today's findings on direct debits that there are areas of the market where customers are simply not getting the service they need and rightly expect in these very difficult times."
British Gas, EDF, Scottish Power and SO Energy were given a clean bill of health. Bulb, E.ON, Octopus Energy, Outfox the Market, Ovo, Shell and Utility Warehouse were found to have some minor issues.
https://www.bbc.co.uk/news/business-62148487