Around 300 Spar stores have had to close due to a cyber attack, the firm has said.
The issue started yesterday, and meant Spar stores were forced to close as they could not take card payments.
The website for Preston food distributor James Hall and Co, which sends goods to nearly 600 Spar shops throughout the north of England, crashed yesterday.
A Spar spokesperson said: "At James Hall we are currently aware of an online attack on our IT systems.
"This has affected around 330 Spar stores across the north of England over the past 24 hours and 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."
The chain was still affected by the issue on Monday morning, Spar said on Twitter.
Is your local Spar closed? Let us know in the comments below
A Spar store near Preston, posted that there has been an "widespread IT failure across the entire Northern Spar network" with all stores closed for an "unknown period of time".
It posted: "Due to a major and widespread IT failure across the entire Northern Spar network, all Northern Spar stores will be closed for an unknown period of time. We will update when we know more, but in the meantime please understand that our staff cannot open until the systems are restored.
"At this stage no-one knows if that will be 10 minutes, 10 hours or 10 days."
One Spar customer said on social media: "Anyone know what happened to Spar shops today? Saw an attendant who said the whole till system has been down all day in all UK Spars. Very odd."
Other Spar stores also reported problems with their IT systems.
Spar stores are run as franchises, and one operator is Lawrence Hunt & Co Ltd, which runs 25 branches, Lancashire Live reported.
Yesterday the firm said: "Unfortunately due to a total network outage affecting tills, credit cards and back office systems, it looks like we will remain closed for the whole of Sunday - we hope our partners will get us back up and running very soon."
Some Spar operators said the issue was caused by a security breach.
Due to a security breach on the network system, our SPAR shop is currently unable to trade and therefore closed. We are working with SPAR to rectify the situation and we will let you know as soon as we are able to re-open. pic.twitter.com/xDdx77TjJ1
— Hull Uni Students' Union (@HullUniUnion)
Last week The Mirror reported that shoppers can now get cashback in many Spar branches without making a purchase.
Spar is joining retailers like Nisa and Londis to offer the new service, under plans being rolled out by cash machine network LINK.
The “Cash Without Purchase” scheme sees customers able to withdraw up to £50 in notes or coins without needing an ATM.
To take part in the scheme, shops will need to have a PayPoint terminal installed.
