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.
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.
Visa users received messages telling them there is a "disruption of service", meaning payment providers are unable to receive or send money.
A range of different banks and other financial institutions use Visa's payment system, meaning that even transactions using non-Visa branded cards could be affected by the problem.
The issues have been described as intermittent, and some payments over the Visa network do still appear to be going through. Cash withdrawals do appear to be functioning.
A statement from Visa suggested that it didn't yet know the cause of the outage.
“Visa is currently experiencing a service disruption," it read.
"This incident is preventing some Visa transactions in Europe from being processed.
"We are investigating the cause and working as quickly as possible to resolve the situation.”
