The technology meltdown affected the Royal Bank of Scotland (RBS) Group and led to customers being unable to pay for goods over the internet and in stores.
There were also reports that the problem was affecting the websites and smartphone apps run by the group, which includes NatWest and Ulster Bank.
On a day dubbed Cyber Monday, which is typically when the highest amount of online transactions are expected so shoppers can meet pre-Christmas delivery deadlines, customers took to social media to vent their frustrations at the bank.
RBS customer Steven Mercer tweeted the bank directly, saying: "The shame of doing your Christmas shopping only to be told that your card has been declined, due to your incompetent systems."
An RBS spokeswoman said: "We are aware of the technical issues being experienced by customers and are working to get them fixed urgently," she said. "We apologise to customers for the inconvenience caused."