Visa card payments are failing across the UK and Europe.
The company said it is working towards resolving the problem.
Visa users have taken to social media to complain, and it is understood the Bank of England is aware of the issue and is in touch with Visa.
A Visa spokesman said: "Visa is currently experiencing a service disruption.
"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."
HSBC's UK Twitter account replied to a customer query on Twitter: "We believe there may have recently been issues with Visa's payments processing.
"From what we understand there are still intermittent issues but services are slowly recovering."
Alex Neill, Which? managing director of home products and services, said: "Clearly this issue will be a huge inconvenience to customers and it must be resolved urgently.
"Visa and the banks need to ensure no-one is left out of pocket due to this outage.
"We strongly advise people to keep any evidence of extra expenses they've incurred in order to claim them back."