Tesco has apologised to customers after a glitch led to it taking up to three months to process hundreds of credit and debit card payments.
The UK's biggest retailer said approximately 300 of its 1,700 Express convenience stores had been affected by the problem with its payments system since the end of November.
A Tesco spokeswoman said: "As soon as we identified this issue, we contacted as many affected customers as possible and have now processed all incomplete transactions.
"The issue has now been resolved and we are sorry for any inconvenience this may have caused."
Letters sent to those affected said: "We are writing to let you know that a number of credit and debit card payments did not process correctly for some customers, in a small number of our Express stores.
"You may have noticed this as an unexpected transaction at Tesco in your bank statement."
MoneySavingExpert.com first reported the error and advised customers to check their statements carefully.
It said a number of customers had reported that the delayed transactions had led to them going beyond their agreed overdraft limits.
Tesco has agreed to refund any bank charges incurred as a result of unexpected overdraft usage, subject to proof.
Steve Nowottny, news and features editor at MoneySavingExpert.com, said: "It's good to see Tesco has apologised to shoppers, but this is a real blunder. For a major supermarket to take payments up to three months late simply isn't acceptable.
"If you're operating on a tight budget it's crucial you know that payments will be processed more or less as you make them."