Energy suppliers are to refund customers on up to 1.5 million pre-payment gas meters after a fault led to "over-collection".
Trade association Energy UK said suppliers will be contacting every affected customer and will refund any money owed "as quickly as possible".
It said the problem came about because up to 1.5 million meters did not work properly when they were installed, adding that suppliers "know who is affected and have a solution".
Energy UK said in a statement: "As soon as the issue came to light companies began work to fix it and arrange refunds.
"In some cases a customer's meter will have already been fixed as programmes get under way across the country. All the companies involved will be working over the coming months to get round everyone who is affected."
Pre-payment meters use a calorific value (CV) code to calculate the amount of gas to be charged for.
But the fault meant the CV for some meters was not set properly, resulting in inaccurate readings.
Energy UK said affected meters would be fixed in a roll-out starting today, and the first of the refunds were expected to be given out before Christmas.
It said the industry recognised the distress this could cause and was working "as fast as it can" to resolve the problem, adding that the amounts were "very small, and most top ups will not have been out by more than 25 pence".
Angela Knight, chief executive of Energy UK, said: "Suppliers are working hard to roll out a solution for an issue which has affected some pre-payment gas meters.
"The meters were manufactured with a problem where they over-collected from customers. However, the companies know who is affected and will be getting in touch directly to put the meters right and refund any money owed.
"The suppliers and the wider industry are all very sorry this problem has occurred but are already getting in contact with customers. Customers do not need to do anything - their supplier will get in touch but, if they are concerned, they just need to contact their energy company who will be able to help."
Which? executive director Richard Lloyd said: "This is yet another shameful example of energy companies failing their customers and one that has hit some of the very people who can afford it least.
"Suppliers need to fix this fault as soon as possible and fully reimburse - with interest - the hundreds of thousands of people who have been overcharged.
"We urgently need major reforms to fix this market and increase the low levels of trust we consistently see in energy firms."
