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 27 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."