Customers across the UK and Europe have been unable to pay for their purchases with Visa cards because of a network failure.
Some shoppers reported they were stuck in queues in supermarkets and petrol stations as Visa transactions could not be processed.
Banks have advised customers that they can still withdraw cash from ATMs.
Visa says it is working to resolve the issue. Mastercard and American Express said they were not affected.
Payment processing through Visa’s systems accounts for £1 in £3 of all UK spending, the company said.
Paymentsense, which runs card services for small businesses, said some transactions were now starting to go through but there was a backlog.
Responding to complaints on social media, HSBC said there were issues with processing Visa payments, which were intermittent, but were “slowly recovering”.
Jay Curtis, from Swansea, had two cards declined in B&Q when he tried to pay for £240 worth of goods.
“My card just wouldn’t go through,” the 32-year-old told the BBC.
“I didn’t have cash on me so I had to drive all the way home.”
Labour MP Angela Rayner seems to be among those affected, tweeting that she had to leave her local petrol station without paying.
Elle Gibbs-Murray, from Bridgend, said she was stuck in traffic on the Severn Bridge for 45 minutes as drivers were unable to pay the toll by card.
Adam, from Manchester, is a on a canal boat holiday with his girlfriend, Rach, and he is unable to use his card.
The 26-year-old said: “We have spent all day boating to moor up at a riverside pub in Kidlington for a birthday meal only to find the visa payments are not working. Having only £20 between us we have had to opt for a birthday drink instead.
“[There is] no cash point for miles around and no car as we are on the canal boat.”
In Berlin’s Alexanderplatz, customers at Primark complained of having to queue for 20 minutes to pay and staff there could not explain the reasons why transactions were failing.
Deborah Elder, from Glasgow, was unable to pay her restaurant bill while she was waiting at Frankfurt airport to fly back to Toulouse.
She said: “I was so embarrassed. I gave the waiter the 14 Euros I had left.
“I’m worried I won’t be able to get home when I land in Toulouse as I have no cash for a taxi.”
Supermarket Tesco said chip and pin payments were not affected, but contactless payments were.
Sainsbury’s also said it was experiencing problems.
Banks, including the Bank of Ireland, have said cash withdrawals at ATMs can still be made.
Consumer advocacy group Which? advised people to keep evidence of extra expenses incurred in order to claim them back in the future.
“Visa and the banks need to ensure no-one is left out of pocket due to this outage,” said Alex Neill, Which? managing director of home products and services.
The Bank of England, which oversees electronic payment providers such as Visa, said it was “aware of the issue” and was “working with the firm to resolve the situation”.
Transport for London said it expected customers to still be able to use their contactless payment cards to pay for travel on its buses, trains and Tubes.
However, Paymentsense, which runs card services for small businesses, said transactions were now starting to go through but there was a backlog.
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