A major flaw in the way some gas meters are read is an industry-wide problem that may have affected as many as 11,000 energy customers, with many having been overcharged and owed a refund by suppliers, it was revealed today.

In some cases, customers' metric meters have been recorded as being imperial, resulting in them paying too much, while in others imperial meters have been read as metric, meaning users were undercharged.

The full scale of the problem was disclosed today by Energy UK, an industry body which has been investigating the blunder since 'big six' supplier E.on first spotted it last week. Energy UK said it now understood the issue was "not a problem that affects one company in particular".

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How widespread is the problem?

Energy UK has so far refused to reveal which energy suppliers other than E.on have over- or undercharged customers. But it says it believes it's happened in between 10,000 and 11,000 cases.

When E.on first reported the issue last week, it said some customers had been overcharged by as much as 183%, and others undercharged by as much as 65%, with some people having been incorrectly charged for as long as 15 years. We don't yet know the extent of over- or undercharging at other suppliers or how long it's been going on for.

How do I know if I'm affected?

Energy UK says anyone who's affected will be contacted directly by their supplier.

But if you want to check if your gas meter is based on metric or imperial measurements, you can often do this just by looking at it:

  • An imperial meter will usually say "ft³" somewhere on the front of it, while a metric one will normally say "m³".

  • With a metric meter partial units of gas are highlighted in red and those digits are often separated from the other numbers by a decimal point. With an imperial meter the partial units are often displayed on a dial. See the Gov.uk website for pictures which show these differences.

What if I've been incorrectly charged?

That depends on whether you've been over- or undercharged:

  • If you've been overcharged... Energy UK says your supplier will contact you and refund what you overpaid plus an "appropriate" amount of interest by the end of October.
  • If you've underpaid... you won't have to pay the difference though. Undercharged customers in vulnerable circumstances may receive support from their supplier to help ease the transition to accurate bills.

What if I've recently switched to a different supplier?

If the account you were incorrectly charged on is still open, it'll be easy for energy suppliers to contact you and let you know you've been affected. The problem comes with people who have overpaid on old, closed accounts and are now due a refund. In those cases, the former supplier might not have your current contact details.

If you're concerned you might have overpaid on an old account, you should contact your former energy provider or get in touch with Citizens Advice.

Up to 11,000 hit by gas metering blunder – are you affected?
Energy UK has not said which suppliers have been charging customers the wrong amount

What does Energy UK say?

The industry body published the following statement on its website today: "A very small number of customers have been charged the wrong amount for their gas, due to the incorrect recording of imperial and metric gas meters.

"Under 11,000 people (out of 23 million accounts) have either been charged too much or too little.

"Industry has signed up to a set of principles to redress those affected. Where there has been an overcharge, a refund will be issued as soon as possible."

What does Ofgem say?

An Ofgem spokesperson says: "When things go wrong, Ofgem expects suppliers to act quickly to put things right. Suppliers have moved quickly to identify the small number of customers affected by this gas meter billing issue.

"Ofgem welcomes suppliers' promise to refund customers who have been overcharged promptly and will ensure that they receive repayment as soon as possible."