The payments look to customers like they are being declined, in the same way as when a card has insufficient funds. But the issues are actually a consequence of the global network of payments run by Visa
The Visa payment system has crashed, leaving some people unable to buy things or complete transactions.
Even shoppers not using Visa have been unable to make purchases, because the network provides payment systems for a range of shops and financial institutions.
Customers have reported arriving at tills to have their cards declined. Retailers say they have been left unable to take payments in shops, bars and other outlets, forcing them to resort to only taking cash or not making sales at all.