The error is good news for those affected – they won’t have to pay the correct price for the affected period.

Several thousand Octopus customers have received emails saying they have been charged just 0.1% of the amount of gas they have used over the last past few months. This means they have potentially saved hundreds over the cold winter period.

In the email, Octopus told customers that it “had a confession to make” and informed them of the “unintentional discount”.

The email said the issue had something to do with “reading factors” and “unit flags” which meant customers had only been billed a tiny fraction of the gas they had used.

Octopus said the error affected one particular type of gas meter – electricity payments weren’t affected. Customers that have been paying their direct debits as normal are now hundreds of pounds in credit with the supplier. However, Octopus warned that their bills would be more accurate in future.

Octopus said the error related to customers with old gas meters which run with imperial measurements rather than metric ones – causing a temporary system error.

The mistake covered a period of a few months over winter and up to the most recent bill before the email it sent out last week. It said it will honour the discount up to the last time the affected customer was billed.

An Octopus Energy spokesperson said: “We spotted a system error affecting a very small group of customers with older gas meters which still used imperial measurements.

“The customers in question got a 99% discount on their gas usage for a short period of time over winter, which we have honoured up to their latest bill. Customers don’t need to worry, the error is now fixed and all those affected have already been contacted.”