WH Smith has confirmed it has been the target of a cyber attack in March in which hackers accessed company data.
The high street giant added criminals have got into current and former employee information.
The company said the hack has not impacted its trading activities and stressed its website, customer accounts and customer databases are on unaffected separate systems.
A statement from WH Smith said: “Upon becoming aware of the incident, we immediately launched an investigation, engaged specialist support services and implemented our incident response plans, which included notifying the relevant authorities.”
It added: “WH Smith takes the issue of cyber security extremely seriously and investigations into the incident are ongoing.
“We are notifying all affected colleagues and have put measures in place to support them.”
The incident comes in a wave of cyber attacks in recent months, with Royal Mail’s international postal service suffering lengthy disruption after hackers targeted the group.
Retailer JD Sports warned earlier this year around 10 million people might have had their addresses, phone numbers and email addresses stolen – among other things – in a hack.