WM Morrison Supermarkets PLC (LON:MRW) said it would appeal a High Court ruling that found it was liable for a data breach by a disgruntled former employee.
The High Court ruled the supermarket was to be held responsible for the actions of former auditor Andrew Skelton, who stole the data of nearly 100,000 staff and posted it online.
Skelton was jailed for eight years in 2015 over the leak, which included names, addresses, salary and bank account details.
Those affected will be able to claim compensation for the “upset and distress” caused by the incident, the court heard.
A second trial will now be held to determine the amount Morrisons must pay in damages.
Nick McAleenan of JMW Solicitors, who represented the 5,518 claimants, said it was a landmark decision and the first data leak class action in the UK.
He said: “Every day, we entrust information about ourselves to businesses and organisations. We expect them to take responsibility when our information is not kept safe and secure.
"The consequences of this data leak were serious. It created significant worry, stress and inconvenience for my clients. Data breaches are not a trivial or inconsequential matter. They have real victims. At its heart, the law is not about protecting data or information – it is about protecting people."
Morrisons denied any wrongdoing and believes it should not be held responsible for the actions of a former employee.
In a statement, it said: "A former employee of Morrisons used his position to steal data about our colleagues and then place it on the internet and he’s been found guilty for his crimes. The judge found that Morrisons was not at fault in the way it protected colleagues’ data but he did find that the law holds us responsible for the actions of that former employee, whose criminal actions were targeted at the company and our colleagues."
The company added that it had worked to get the data taken down quickly, provide protection for the employees affected and reassure them that they would not be financially disadvantaged.
"In fact, we are not aware that anybody suffered any direct financial loss."
Shares in Morrisons fell 1.01% to 214.11p in morning trading.