Advertising giant WPP said the firm had been hit by a "suspected cyber attack" amid reports hackers had caused widespread disruption to companies across the globe.
The world's biggest advertising business said a number of its companies had been affected and it was currently "assessing the situation".
While the FTSE 100 firm gave no more detail about how the business had been impacted, its website appeared to be down as it made the announcement.
In a statement, WPP 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."
It came as company and government officials reported major disruption to the Ukrainian power grid, banks and government offices.
Russia's Rosneft energy company also reported falling victim to hacking, as did shipping company AP Moller-Maersk, which said every branch of its business was affected.
Ukrainian deputy prime minister Pavlo Rozenko posted a picture of a darkened computer screen on Twitter, saying the computer system at the government's headquarters has been shut down.
There is very little information about who might be behind the disruption, but technology experts who examined screenshots circulating on social media said it bears the hallmarks of ransomware, the name given to programs that hold data hostage by scrambling it until a payment is made.