Energy firms have been ordered to refund thousands of gas customers affected by a meter reading mistake.
The error - caused by companies confusing measurements from older imperial meters with modern metric ones - is believed to have affected about 10,000 customers.
"We have written to suppliers and asked them to refund affected customers," an Ofgem spokesperson told the BBC.
Trade body Energy UK said firms were "working hard" to address the issue.
The trade association, which represents the major energy companies, said "detailed analysis" showed "an extremely small number of accounts" had been affected.
"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," it added.
The energy regulator Ofgem is expected to issue a formal statement on Monday.
The mistake has led to some customers being overcharged and some undercharged, according to the Financial Times, which first reported the story.
The revelation comes just days after Ofgem promised "to deliver a more competitive, fairer energy market for all consumers" as part of a market shake-up aimed at helping consumers save money.
In July, a two-year inquiry into the energy market by the Competition and Markets Authority (CMA) found that 70% of domestic customers using the big six energy suppliers were on expensive default variable tariffs.
As a result it said that such consumers could save £300 a year by switching. Overall consumers were paying £1.4bn more than they should be, it found.
Energy firms have previously been fined for billing errors. Last year, Npower was ordered to pay a record £26m settlement for billing issues affecting more than 500,000 customers.