Energy companies have been ordered to refund thousands of customers affected by a mistake in gas meter readings.
The error reportedly occurred when imperial meters were confused with metric ones, the Financial Times (£) reported.
More than 10,000 people are believed to have been affected.
News of the error emerged just days after Ofgem promised a market shake-up to save consumers money.
The older meters measure usage in cubic feet whereas newer ones use cubic metres.
Some customers are likely to have been undercharged if their metric meters were read as imperial, while others will have been overcharged if the reverse happened.
Industry sources told the FT that some people may have underpaid by 60% as a result of the error but those who were overcharged may have paid in excess of 130% more than they should have.
The industry's regulator Ofgem said it was working to ensure that anyone who had been overcharged was contacted and repaid the cash.
"Ofgem is aware that a very small number of gas customers are being incorrectly charged for their energy use and has taken fast action," it said in a statement.
"Working with Energy UK, we’re ensuring suppliers take immediate and decisive action to identify affected customers."
Energy firm E.ON - one of the 'big six' providers - said it had identified 600 customers who had been incorrectly billed as a result of the confusion.
It said that 350 households and businesses which had been overcharged will get refunds, while the 250 who were undercharged will not have to pay any money back.