THOUSANDS of Powergen customers could be getting backdated electricity bills because their details were not properly recorded when they switched energy supplier.
Customers affected are those who were formerly served by TXU Energi in the east of England. Powergen took over TXU in 2002.
A Powergen spokesman said he was unable to say exactly how many customers were affected, but that it was less than 20,000.
"In the vast majority of cases where there's an error we're talking about less than two years," he said.
Powergen is now examining the transferred TXU accounts to work out exactly how many of their customers are affected.
The spokesman added:
"We're contacting customers as we look at these accounts and we're asking customers to contact us."
The spokesman said that compensation would be considered.
He said: "Errors do occur. We're talking small numbers. We've got six million customers. We see this as a one-off."