UK supermarkets Morrisons and The Co-op have suffered payments glitches that have left some customers unable to pay by card and others charged twice.
Co-op stores began experiencing problems on Sunday, leaving some customers forced to queue outside in the snow and to pay with cash.
ACI, which processes payments for both retailers, tells the BBC: "We are working closely with the IT teams at both partners to resolve the problem as quickly as possible. We apologise to shoppers for any inconvenience caused."
The Co-op says the "intermittent" issue is affecting only a "very small" number of transactions but customers at some stores were told that they could only use cash and self-service checkouts were closed.
Other shoppers have taken to Twitter to say that they have been charged twice:
Shadow economic secretary to the Treasury Pat McFadden says: "This shows the dangers of letting the cash network just wither away as use declines.
"The government promised legislation to secure nationwide access to cash a year ago. It hasn't been brought forward."