Selfridges suffered downtime on the payment card processing systems it uses in its London store last week.
On Thursday afternoon, shop staff at tills across the store were unable to accept payment by either credit or debit cards.
One shopper told Retail Week that customers were initially being redirected to other tills in the store by staff who did not realise that the whole system was down.
A spokeswoman for the retailer said that problem had been remedied by Friday morning and the store was once again operating as normal.
She was unable to comment on what caused the problem and declined to give a figure for the reduction in trade it suffered as a result of the system downtime.