Visa has admitted 5.2 million card transactions were affected by an IT system failure earlier this month, including 2.4 million in the UK.
The company promised “appropriate compensation” for customers hit by the disruption, which lasted just over 10 hours.
During that period more than 27 million transactions were made in the UK, of which 9% failed to process.
Visa said on Tuesday that the issue was caused by a “very rare partial failure” of a switch in one of its data centres, which meant its secondary centre could not automatically process all transactions.
It has since fixed the issue and has pledged to compensate users “if deemed necessary” by issuing banks in the UK.
Visa has also commissioned EY to conduct an independent review of the incident and the company’s response.
In a letter to Treasury select sommittee chair Nicky Morgan, Visa Europe chief executive Charlotte Hogg said: “In this instance, we failed to meet your and our own expectations and we apologise again unreservedly to everyone affected by the incident.
“Visa, together with our financial institution partners, has quickly implemented a compensation programme for cardholders in response.”
Morgan said: “The Treasury committee is satisfied with Visa’s answers regarding its system failure earlier this month, which lasted just over 10 hours and saw 2.4 million transactions in the UK fail to process. It appears that the problems have been fully resolved.
“The news that debit card payments have overtaken cash use for the first time shows that the reliability of IT systems is becoming ever-more important.
“The detriment caused to consumers by IT failures is greater than ever, so the Committee will become less tolerant of them.
“The committee expects to see the findings of the independent review, which will examine the lessons to be learned from the incident, in full.”