Millions of TSB customers were locked out of their accounts during an IT “upgrade” leaving some people with unexpected credits and debits on their accounts.

TSB said its 1.9 million mobile and internet banking customers were affected during a transfer of banking data involving the switch of 1.3 bn customer records, which was due to be completed by Sunday evening. However on Monday TSB received complaints that customers were unable to log on.

A spokesperson for TSB said if any customer was left out of pocket as a result of the IT transfer they would be reimbursed.

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