Irish carrier Aer Lingus has been caught up in a cyber-attack which has hit organisations around the world, including sister IAG airline British Airways.
BA wrote to UK employees on Monday (June 5) to inform them that payroll company Zellis had been hit by a data breach.
Hackers are understood to have taken advantage of a flaw in a tool used by Zellis, which serves BA and many other major businesses.
Other companies affected by the hack include the BBC, Boots, Nova Scotia Government and the University of Rochester, according to the BBC.
Reports said that about 5,000 Aer Lingus employees have been affected, as well as former staff.
The BBC said that a “prolific cyber-crime gang thought to be based in Russia” has now issued an ultimatum to victims of the hack.
It reported: “The Clop group posted a notice on the dark web warning those affected by the MOVEit hack to email them before 14 June or stolen data will be published.”
Employers are being urged not to pay up if the hackers demand a ransom.
“Advice from experts is for individuals not to panic, and for organisations to carry out security checks issued by authorities like the Cyber Security and Infrastructure Authority in the US,” added the BBC.
More: British Airways staff suffer cyber attack targeting bank details