Britain's energy regulator said that thousands of customers affected by a reading mistake on their gas meters will have to be refunded by energy firms.
Ofgem, said it has already spoken to energy providers about the issue and has instructed them to refund the customers who have been affected.
"We have written to suppliers and asked them to refund affected customers," an Ofgem spokesperson said.
"Ofgem is aware that a very small number of gas customers are being incorrectly charged for their energy use and has taken fast action. Working with Energy UK, we're ensuring suppliers take immediate and decisive action to identify affected customers."
The error, which is understood to have affected approximately 10,000 consumers, has been caused by companies confusing measurements from older imperial meters with modern metric ones. Ofgem said E.ON is so far the only energy provider to have come forward and admitted the issue.
"Any customer affected will be contacted shortly by their supplier and where there has been overcharging, a refund will be issued as quickly as possible," said trade body Energy UK, adding firms were "working hard" to rectify the mistake.
According to Energy UK, only "an extremely small number of accounts" had been affected.