Visa went gone down across Europe on Friday, causing chaos for customers as retailers were left unable to take payments.
People heading out as the weekend began were being advised to take cash with them as supermarkets, restaurants, and bars across the UK and continent were affected.
Late Friday night Visa announced it was back up and operating “close to normal levels”.
Users took to social media to complain, and it is understood the Bank of England was aware of the issue and in touch with the company.
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.”
There was anecdotal evidence of some people taking advantage of the situation...
Visa said in a statement: “Earlier today, Visa had a system failure that impacted customers across Europe.
“Our goal is to ensure all Visa cards work reliably 24 hours a day, 365 days a year. We fell well short of this goal today and we apologise to all of our partners, and most especially, to Visa cardholders.
“Visa cardholders can now use their Visa cards as we are currently operating at close to normal levels.
“The issue was the result of a hardware failure. We have no reason to believe this was associated with any unauthorised access or malicious event.”
The Visa network is relied upon worldwide to process payments.
In 2014 it processed 100 billion transactions with a total volume of just over £5 billion.