IT systems targeted

Europe were targeted – Ukrainian officials reported serious “intrusions” at the country’s power grid, banks and government offices.

Ukrainian prime minister Volodymyr Groysman described the cyberattack as “unprecedented” , although vital systems were unaffected.

Pavlo Rozenko, the country’s deputy prime minister, posted a picture of a darkened computer screen on Twitter, saying the network at the government’s headquarters had been shut down.

Cyber security experts suspected a form of ransomware, the name given to programmes that hold data hostage by scrambling it until a payment is made.

It is only a few weeks since the last incident involving ransomware, called WannaCry or WannaCrypt, spread rapidly around the world using digital breakin tools originally created by the US National Security Agency.

More than 200,000 victims in around 150 countries – including the NHS – were infected by the ransomware, which originated in the UK and Spain in May before spreading globally.

WPP said a number of its subsidiaries were affected by the latest cyber attack and it was “assessing the situation”.

Global law firm DLA Piper, which has offices in London and other parts of the UK, revealed it had also been affected.

US pharmaceutical company Merck said: “We confirm our company's computer network was compromised today.”

The UK’s National Cyber Security Centre, part of intelligence agency GCHQ, confirmed there had been a “global ransomware incident”.