More than 300 stores have been affected by the attack, although not all have closed.
Those that remain open are only accepting cash due to debit and credit card payments not being able to operate.
This content is not available due to your privacy preferences.Update your settings here to see it.
The retailer said the attack had hit all of the company’s IT systems, including staff emails.
The IT outage first hit on Sunday and technicians have been working through the night trying to fix the problems.
Distribution partner James Hall & Company, which provides food to nearly 600 Spar sites across the north of England, has also been hit, and its website is down.
Spar said: “We are working to resolve this situation as quickly as possible.
“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,”Internet Explorer Channel Network