Shoppers at Morrisons and the Co-op have been struggling with card payments due to IT issues which are under investigation.
Some shoppers were left queuing in the cold outside Co-op on Sunday and the problems continued into Monday, although the retailer said the issues were intermittent and only affected a very small number of customer transactions.
Consumer group Which? said the issue shows why cash is still a “vital back-up” payment method when technology fails.
Both retailers said on Tuesday that card payments are now being processed as usual, although the cause is being looked at.
Some Twitter users reported that they had been charged for the same Morrisons shop twice while the glitches were happening.
Twitter users also described long queues in Co-op and customers having to dash over to an in-store cash machine in order to be able to pay.
Earlier, ACI, which processes payments for the retailers, apologised for the inconvenience caused.
ACI said: “Customer relationships and the stability of our systems remain an absolute priority for ACI Worldwide. We apologise to shoppers for any inconvenience caused.”
A statement from Morrisons on Tuesday said: “We are no longer experiencing issues with card payments and are working very closely with our partner ACI to investigate what happened.”
And a Co-op spokeswoman said on Tuesday: “All card transactions are being processed as usual and our payment process partner is investigating after we experienced an intermittent issue.
“We would like to apologise to customers for any inconvenience caused during that time.”
The BBC reported that the issue in some Co-op stores meant self-service checkouts had to be closed, requiring customers to queue to be served at tills manned by staff.
It said long queues were seen outside some of the Co-op’s convenience stores from Sunday amid the snow, and customers were encouraged to pay using cash.
IepMu local Co-op too. Staff said it's been a problem all day. Customers had to dash over to use the in-store cash machine then pay in cash. Long queues!!Strangely, after my Barclays Debit Card was "declined" I tried my Smile Card and that worked first time!
— David of Nottingham (@DavidofNotting1) January 25, 2021
Many shoppers have been using cards and contactless payments during the coronavirus pandemic in preference to cash, although a recent Bank of England study concluded the risk of catching coronavirus from banknotes is low.
Gareth Shaw, Which? head of money, said: “This was a concerning situation for shoppers and reinforces why cash is such a vital back-up when technology fails – even for those who prefer to use digital payments.
“A reduction in retailers accepting cash during the pandemic has combined with widespread bank branch and cash machine closures to push the UK’s already fragile cash system even closer to the brink of collapse.
“To avoid cutting off millions of people from their preferred payment method, the Government must urgently introduce the legislation promised in last year’s Budget and the industry must commit to supporting the existing cash network until a new system is in place.”