Thousands of householders have been overcharged on their gas bills due to imperial meters being mistaken for metric ones.
Ofgem has called for gas suppliers to reimburse any customers affected by the error, after the problem was discovered by E.On.
The energy giant admitted they had overcharged 350 of their customers, with one householder paying too much for 15 years.
Compensation would be calculated on each account individually, with some being overcharged a massive 65% more than they should.
Some householders have been undercharged, but will not have to pay extra.
The mistake occurred when imperial readings were mistaken for metric ones, with several thousand people affected.
All the other energy companies have been ordered by Ofgem to identify any customers who may have been overcharged by the end of the week.
The mistake was a result of “human error” with the customers affected being refunded in full including any interest and compensation deemed appropriate.
Energy UK have said that all firms were “working hard to address the issue” with “detailed analysis showing an extremely small number of accounts” affected by the issue.
“Any customer affected will be contacted shortly by their supplier and where appropriate a refund will be issued as quickly as possible.”