The Works has been forced to close some of its stores, after a cybersecurity incident caused issues with its tills.
The stationery and books retailer said the hack also meant delivery of new stock had been temporarily suspended and had caused longer delivery times for online orders.
It said store deliveries “are expected to resume imminently” and its normal online services are steadily being reintroduced.
The retailer is also reassuring customers they can safely shop in stores that have remained open, and that it is safe to pay by card on their website.
In a statement, The Works said: “There has been some limited disruption to trading and business operations, including the closure of some stores due to till issues.
“Customers can continue to shop safely at The Works, both in store and online.
“All debit and credit card payment data are processed securely outside the group’s systems, via accredited third-party networks, and therefore there is no risk that this payment data has been accessed improperly.”
The retailer said it has taken a number of actions after it was alerted to the breach by its security firewall.
The Works has disabled all internal and external access to its systems, including email, while it works with its advisers to evaluate and rectify the situation.
External cyber security experts have also been appointed and are currently completing investigations and recovery work.
The company added that, while payment data has not been compromised, it has not yet established “the full extent to which any other data may have been affected”.
It said it has therefore informed the Information Commissioner’s Office.