Furious customers of energy provider E.On received an unwelcome Christmas Eve surprise when their bill payments were taken too early.
Direct debits that should have been taken from customers in the next two weeks were taken on Thursday.
About 1.5 million customers – including householders and small businesses – have been affected.
The company apologised and said refunds would be made on the first available date, which was 29 December.
Many customers said they were facing Christmas overdrawn as a result of the company’s IT error.
Others told the BBC that they faced the added frustration of being unable to contact the company for an explanation.
An E.ON spokeswoman said: “Due to an IT issue, we have inadvertently taken direct debit payments early from some of our customers.
“We are sorry for this error and are taking steps to contact affected customers where we can, as well as putting information about the issue on our website and social media channels.
“Customers do not need to do anything or contact us, and we ask that they bear with us while we work to refund them on the first available date, which is 29 December. Customers’ direct debit payments will then be taken in line with their usual payment schedule.
“If a customer has incurred bank charges as a result of this issue, we will of course reimburse this money to them. Any customer who is concerned should contact us to discuss their circumstances.”
The company said that if customers were unable to wait for a refund, they should contact their bank directly for possible support.