A rare malfunction of hardware in one of Visa’s data centre was the cause of the outage that left millions of people across Europe unable to make card payments on 1 June.

In an 11-page letter to Nicky Morgan, chair of the Treasury Committee, Visa detailed the reasons for the system failure, its effect on the UK and Visa’s response and plans for avoiding such problems in the future.

In total, 5.2 million Visa transactions failed to process correctly across Europe, with 2.4 million of those in the UK. During the period of disruption, 1.7 million UK Visa cards were impacted – representing 10.4 per cent of total cards used during the outage.

Visa explained that the incident was caused by the failure of a switch in one of its European data centres, and are running a forensic analysis with the manufacturer on why the switch failed when it did.

Charlotte Hogg, chief executive of Visa Europe, explained: “In this instance, the switch being used in our primary data centre experienced a very rare, partial failure, which impacted the secondary site and prevented it from automatically processing all transactions, as it should have.

“As a result, it took far longer than expected to isolate the primary site; in the interim, the malfunctioning system at the primary data centre continued to try to synchronise messages with the secondary site. This created a backlog of messages at the secondary data centre, which, in turn, slowed down that site's ability to process incoming transactions.”

Hogg stated that Visa will ensure that those customers who incurred additional expenses through the outage would be appropriately compensated. Visa is also waiving fees to UK acquirers for transactions which failed to process.

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