For a period of time the WPP.com website was inaccessible although a statement on the site said it was due to ‘routine maintenance’.

However WPP did issue a statement towards the end of the day yesterday confirming that "some of its information technology systems have been affected by a global cyber-attack. WPP is assessing the situation and is taking appropriate measures." 

It was affected by aPetya ransomware attack that also hit other organisations around the world.

Subsequently, WPP CEO Sir Martin Sorrell wrote to its employees to reassure them that it appeared no employee or client data had been compromised. He said: “We are working with our IT partners and law enforcement agencies to assess the situation, take all precautionary steps and return to normal operations as soon as we can.”

He added in a statement: "Our operations have not been uniformly affected, and issues are being addressed on a company-by-company basis. Many of our businesses are experiencing no or minimal disruption."

It is thought the attack affected Windows computers but not Apple Macs and mobiles.

The WPP attack follows a ransom-ware attack on the UK’s National Health Service last month.