At least three multinationals based in Ireland have been hit by the worldwide 'Petya' ransomware attack.
The virus started in Ukraine and has spread across the world, affecting banks, government IT systems and energy companies.
Here, the pharmaceutical company MSD which has operations in Cork, Carlow, Tipperary and Dublin says its network was infected.
Danish shipping giant Maersk confirmed computers at its Dublin office in Blanchardstown were down.
And the ad agency WPP, which has offices in Dublin, also fell foul of the bug.
— Maersk (@Maersk) June 27, 2017
Computer users in Leinster House are being warned to be careful, although there is no suggestion they are affected.
It seems computers running older versions of Windows are most vulnerable to attack.
Cyber security expert Urban Schrott from Eset Ireland says having anti-virus patches up to date is also essential:
