Data from a staff payroll systems belonging to supermarket chain Morrisons has been stolen and published online, the company has confirmed.
Personal information on thousands of staff, such as names, addresses and bank details, were published on a website on Thursday evening, before Morrisons staff had the data taken offline.
Morrisons CEO Dalton Philips is leading the response to the theft, and the supermarket has now called in police and cyber security experts to investigate the incident.
No customer data is believed to have been compromised, Morrisons said, and work will be done to ensure that employees will not be "financially disadvantaged".
The supermarket chain claims that initial investigations do not point towards an "external penetration" of its systems, meaning the information was likely to be have downloaded by a member of staff.
A note sent to all Morrisons staff this morning said: "We are extremely sorry to inform you that there has been a theft of colleagues' personal information, which was uploaded onto a website.
"As soon as we became aware of this last night we took immediate steps to ensure the data was removed from the website. It was closed down within hours of us being notified.
"The information included names, addresses and bank account details of colleagues. This affects colleagues from all levels of the organisation.
"Our immediate priority is the security of your financial information. We are currently working with Experian and the major banks to ensure that we provide full support and assistance to all affected colleagues. This will include support and advice around protection of your bank account."
It added: "We are very sorry that this has happened. We will ensure that no colleague will be left financially disadvantaged as a result of this theft."
The theft of data is the latest blow for the firm which announced it would restructure its business following a poor financial performance, with plans to sell off some of its online businesses.
The group has recently undergone a substantial refresh of its ageing IT assets, spending hundreds of millions on modernising systems. The retailer has also launched its long awaited delivery service, after partnering with logistics firm Ocado.