Some HSBC customers were unable to get cash from ATMs and carry out debit card transactions yesterday evening, due to hardware glitch.
The problem occurred between 17:00 GMT and 19:00 GMT.
In a statement HSBC said: "For a short period of time on the evening of Sunday 20 May, an IT hardware failure, which supported some ATM and debit card transactions, meant that a small minority of HSBC customers were unable to withdraw cash or pay for goods.
"We worked as quickly as possible to restore both services, however we understand the inconvenience caused to customers and we apologise for the disruption."
It is not known at this stage whether it was HSBC or a third party that was responsible for the hardware failure.
Customers using non-HSBC ATMs - those using HSBC cash machines were not affected - first started to be declined at around 17:00 GMT, and the bank said that the issue was resolved within 45 minutes.
As a "side-effect" of the ATM failure, shortly after HSBC's debit card customers started to have their cards declined at point-of-sale (POS) transactions. This was resolved by 19:00 GMT, with full service restored shortly after, the bank said.
"Although the POS issue took slightly longer to resolve, Visa 'stood in' to accept debit card transactions so the vast majority went through as normal," a spokesperson said.