Card payments are currently failing across the UK and elsewhere in Europe, Visa has said, BBC News reported.
The extent of the disruption is not yet clear but customers in the UK and Europe have been complaining that payments have been denied.
Barclays and Bank of Ireland advised customers to use ATMs to withdraw cash, which appear to still be working.
Visa is working to resolve the problem but one payments firm said transactions were now working intermittently.
The Payment Systems Regulator told the BBC it understood the problem to be limited to Visa card payments only. Mastercard said it was seeing no problems on its network.
Supermarket Tesco said that the problem was not affecting chip and pin payments, but that it was affecting customers using contactless payments.
Responding to complaints on social media, HSBC said there were issues with processing Visa payments, which were intermittent, but were "slowly recovering".
Sainsbury's also said it was experiencing problems.
Jay Curtis, 32, is in Swansea and had two cards declined in B&Q this afternoon.
"I had £240 worth of stuff in my trolley. My card just wouldn't go through. I tried and again and then I tried with another card but it still wouldn't go through," Mr Curtis told the BBC.
"I didn't have cash on me so I had to drive all the way home."
First Bank told customers on Twitter there weren't "any timescales for resolution at the moment".
However, Paymentsense, which runs card services for small businesses, said that transactions were now starting to go through.