Thousands of customers have been incorrectly charged after energy companies made a mistake in gas meter readings.
Customers whose imperial meters, measured in cubic feet, were read as metric, measured in cubic metres, have been overcharged.
Ofgem said the number of affected customers will not be known before the end of the week.
The industry regulator said it has asked all suppliers to investigate if their customers have been affected and urged them to provide their results by August 19.
Energy giant E.ON raised the issue, prompting a wider probe into how many people have been over or undercharged.
In a statement Ofgem said “a very small number of gas customers” had been affected – believed to be around 0.03% of the 21 million customers in the UK.
It added: “Ofgem is working with suppliers to ensure consumers don’t lose out. This includes suppliers setting out plans to redress their affected customers, and to reach a quick resolution.
“Customers do not need to do anything. The very few affected will be contacted by their supplier.”
E.ON apologised to its customers, saying 600 – including homes and businesses – had been affected.
Those overcharged are to be refunded, while the 250 who were undercharged will not have to pay any money back.
The company said the error occurred because some customers with newer metric meters were registered as having older imperial ones, and vice versa.
In a statement the firm said: “E.ON UK has said sorry and confirmed that around 350 residential and small and medium-sized enterprise (SME) customers will be refunded and appropriately compensated after the discovery of a potentially industry wide meter set-up issue.”
It added: “Within the customers it currently supplies, E.ON has identified all those affected by this issue where the affected meter is still in situ and continue to work to identify any historic meters that have subsequently been exchanged that may have been impacted.
“E.ON will contact all customers individually. Customers do not need to take any action.”
Industry body Energy UK said affected customers will be contacted and refunded.
A spokeswoman said: “Energy UK and its members are working hard to address the issue of the incorrect recording of imperial and metric gas meters. So far, detailed analysis shows an extremely small number of accounts have been affected, so customers need not worry.
“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.”
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