Payment systems giant Visa said Tuesday that a massive technical glitch earlier this month had affected 5.2 million card transactions, almost half of which were in Britain.
Visa, revealing the details in a letter to a British parliamentary committee that is probing the matter, said the "rare" disruption prevented many cardholders form making payments in Europe for 10 hours on Friday June 1.
And the company apologised "unreservedly" for the failure and outlined plans for a compensation scheme.
"Overall, for cards issued both in the UK and elsewhere ... 51.2 million Visa transactions were initiated and sent to Visa's European systems for processing," wrote Visa Europe chief executive Charlotte Hogg.
"Of these, 5.2 million failed to process correctly," she added in the letter to Treasury Select Committee chair Nicky Morgan.
The outage was caused by a "very rare partial failure" of a switch in one of Visa's data centres, Hogg added. It has since been fixed.
Visa said there were 27.6 million transactions made in the UK during the disruption, of which 2.4 million failed to process properly.
"At its peak, the disruption affected people in the midst of returning home from work, socialising in restaurants and pubs, and doing end-of-day shopping," Hogg said in the letter published Tuesday.
"We take seriously our important role in supporting financial stability in the UK.
"A disruption to our processing that impacts consumers at any time is unacceptable, let alone during a busy Friday afternoon," she added.
"We apologise again unreservedly to everyone who was affected by the incident," Hogg said.
"Visa, together with our financial institution partners, has quickly implemented a compensation programme for cardholders."
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