Thousands of customers who used a bank card on New Year's Eve could find they were charged twice because of a "technical error" with a payment system.
A fault with the Lloyds TSB Cardnet merchant system meant people who paid for sale shopping or a night out with a debit or credit card may have seen their transactions duplicated.
Around 200,000 people were believed to have been double-charged, Lloyds said.
The bank said cardholders were being reimbursed and there was a helpline for anyone affected.
Lloyds apologised to customers and said the problem had been resolved.
A statement said: "A technical error with our merchant system on 31 December resulted in the duplication of certain payments from cardholders to some merchants.
"Duplicated transactions have been reversed and cardholders are being reimbursed. We would like to apologise to affected individuals for any inconvenience caused.
"A dedicated helpline has been set up to provide those affected with further information.
"Cardholders who believe they have been financially penalised by their bank or card issuer as a direct result of the duplicated transactions are advised to contact us. We will consider all reasonable and evidenced requests for reimbursement."
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