HSBC customers were denied access to their bank accounts at the weekend after an upgrade to a telephone-banking system caused a fault in its database.
Shoppers across the UK trying to use their HSBC Maestro debit cards were unable to withdraw cash from ATMs or had their chip and PIN cards rejected by retailers.
Problems began at 13:00 (GMT) on Saturday 23 February and continued through the peak shopping period until 6pm.
HSBC apologised to its customers saying the fault had arisen after an upgrade of its telephone-banking system.
One customer who had their card rejected said the bank's customer service department told him it had been inundated with calls from angry account holders.
An HSBC spokesman said not all of its Maestro customers were affected and that it only affected a small proportion of HSBC's 15 million customers, adding that the declined transactions would result in minimal loss for the bank.
The spokesman said: "The way customers access the bank — using the ATM, telephone and internet banking — all rely on the same customer database, so if one part suffers problems the effects follow through the system. These are huge systems which are massively complex and they do at times have problems.
"But when you consider the scale of the systems and the number of transactions that are put through then they are remarkably reliable. No customer loss was experienced. We apologise for any inconvenience."
He said HSBC would mount a full review of the incident in order to learn from the fault.
Mastercard, owner of Maestro, confirmed the problem with HSBC's system and said it affected a number of cardholders and retail merchants.
A MasterCard statement said: "MasterCard has investigated and can confirm that there were no issues with our own system at this time. We understand that the root cause has been identified outside of MasterCard and corrective action has been taken to prevent reoccurrence."