HSBC Bank has said that a "hardware problem" caused the transaction processing outage on Bank Holiday Monday, and maintains the fault has now been corrected.
During the busy high street sales day, HSBC customers were unable to use HSBC debit cards at point of sales, and some credit cards were also affected.
ATMs could also not be accessed and many customers had problems with making transactions on internet banking accounts.
An HSBC spokeswoman said, "The hardware problem has now been solved and everything is now working as it should today, but because many online customers are now returning to their accounts, some may be experiencing delays because of the volumes involved."
"The original problem had nothing to do with transaction volumes or new chip and Pin technology," she added.
Read more on Operating systems software
Has Covid-19 made bank branch closures easier to sell?
Astrophysics and music production skills give Bud a different approach: Fintech interview part 16
Lloyds Bank rolls out open banking functionality to mobile app customers
Why do UK banks suffer more software failures than those in other countries?