HACKERS have unleashed a mass cyber attack, causing widespread disruption to governments and companies across the globe.
The 'Petya' ransomware attack has affected companies in countries including the UK, Russia, India, Spain, France, Denmark and Norway.
The radiation monitoring system at Ukraine's Chernobyl nuclear power plant has been hit by the unprecedented security breach.
Britain-based advertising giant WPP is also among dozens of firms reporting issues.
Ransomware is the name given to programs that hold data hostage by scrambling it until a payment is made.
WPP, the world's biggest advertising business, said a number of its companies had been affected and it was currently "assessing the situation".
In a statement, the FTSE 100 firm said: "IT systems in several WPP companies have been affected by a suspected cyber attack.
"We are assessing the situation, taking appropriate measures and will update as soon as possible."
French construction materials company Saint Gobain also said it had been affected.
A spokesman for the firm said: "Along with other big companies, St Gobain has been the victim of a cyber attack.
"As a security measure and in order to protect our data, we have isolated our computer systems."