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 Eastern European disruption, but technology experts who examined screenshots circulating on social media said it bears the hallmarks of ransomware, the name given to programmes that hold data hostage by scrambling it until a payment is made.
The National Cyber Security Centre, which is part of intelligence agency GCHQ, said there was a "global ransomware incident".
A spokesman said: "We are aware of a global ransomware incident and are monitoring the situation closely.
"The NCSC website provides advice to the public and business on how to protect your digital systems."
In a statement on the company's website, Maersk said: "We can confirm that Maersk IT systems are down across multiple sites and business units due to a cyber attack.
"We continue to assess the situation. The safety of our employees, our operations and customers' business is our top priority. We will update when we have more information."
Global law firm DLA Piper, which has offices in London and other parts of the UK, confirmed it had been affected.
A spokeswoman said: "The firm, like many other reported companies, has experienced issues with some of its systems due to suspected malware.
"We are taking steps to remedy the issue as quickly as possible."
It is understood the company has taken its email system down as a preventative measure.
US pharmaceutical company Merck said: "We confirm our company's computer network was compromised today as part of global hack. Other organisations have also been affected.
"We are investigating the matter and will provide additional information as we learn more."
The attack is described as a "massive ransomware campaign that is currently unfolding worldwide", according to Bogdan Botezatu, a senior e-threat analyst at Bitdefender.
He said samples are similar to the GoldenEye ransomware family, which has been circulating in recent months.
Mr Botezatu said victims of GoldenEye malware can be asked to pay a 300 dollar ransom after their hard drive is encrypted by GoldenEye, crashing their computer.
The latest attack comes just weeks after a ransomware attack downed systems across the globe, including the NHS in the UK.
More than 200,000 victims in around 150 countries were infected by the WannaCry or Wanna Decryptor ransomware, which originated in the UK and Spain in May before spreading globally.