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.