Thousands of Morrisons staff are to sue the supermarket for damages following a data leak.
The case at London's High Court on Monday follows a breach of security in 2014 when a former senior internal auditor at the retailer's Bradford headquarters posted the payroll information of nearly 100,000 employees on the internet.
This included their bank, salary and national insurance details, addresses and phone numbers.
The two-week trial, which is only concerned with the issue of liability, involves claims brought by 5,518 current and former staff who allege that Morrisons failed to prevent the leak.
They claim it exposed them to the risk of identity theft and potential financial loss and that Morrisons, which denies liability, is responsible for breaches of privacy, confidence and data protection laws.
Lawyers say that the case, the first data leak class action in the UK, has potential implications for every individual and business in the country.
It is thought to be the biggest-ever claim in relation to a data breach in the UK and comes amid growing concerns about cyber security.
Nick McAleenan, of law firm JMW Solicitors, said: "The court will decide whether Morrisons bears any legal responsibility for the misuse and disclosure of the payroll information of the many thousands of people bringing claims in this case."