
Visa's backup measures in the UK enabled the system to recover three hours after it failed. The company is now carrying out diagnostics.
The credit card giant told customers in an email: "During this [system failure] time period, authorisation requests on some Visa branded cards did not receive any bank response."
Visa admitted that some transactions would not have been processed, but could not say how many. The company said that backup measures had enabled the system to recover.
Europe's 315 million Visa debit, credit and commercial cards were used to make purchases and cash withdrawals of more than £8.1bn in 2006.