More than 300 stores have been affected by the incident, although some have avoided closing by switching to cash payments.
The supermarket chain SPAR has been forced to close a number of stores following what it confirmed on Tuesday was a ransomware attack.
A spokesperson confirmed the nature of the incident to Sky News, but said policy was to not identify the criminal organisation involved.
It followed James Hall and Company saying that more than 300 convenience stores had been affected by the incident, though some had avoided closing by switching to cash payments.
The Lancashire-based company, which provides wholesale and IT services to SPAR stores, said that its staff payroll was unaffected ahead of Christmas.
However the spokesperson added that as some stores operate independently of James Hall then it could not say whether any employees were on zero-hours contracts and thus left unable to work as a result of the incident.
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The spokesperson explained that the ransomware attack had impacted all of the company’s IT systems and left staff without access to emails.
“We are working to resolve this situation as quickly as possible,” the company said in a statement.
“It is currently impacting stores’ ability to process card payments meaning that a number of SPAR stores are currently closed to shoppers or only taking cash payments.”
“We apologise for the inconvenience this is causing our customers and we are working as quickly as possible to resolve the situation,” the statement added.
James Hall did not explain whether they intended to pay the attackers’ ransom demand to access the IT systems again or if they were recovering them from backups.
A spokesperson for the UK’s National Cyber Security Centre said: “We are aware of an issue affecting Spar stores and are working with partners to fully understand the incident.