Firms around the globe are reporting that they have been hit by a major cyber-attack.
British advertising agency WPP is among dozens of firms reporting problems.
Ukrainian firms, including the state power distributor and Kiev’s main airport were among the first to report issues.
Some experts are suggesting that it could be a ransomware attack, similar to Wannacry which hit last month.
Alan Woodward, a computer scientist at Surrey University, said: “It appears to be a variant of a piece of ransomware that emerged last year.
“It was updated earlier in 2017 by the criminals when certain aspects were defeated. The ransomware was called Petya and the updated version Petrwap.”
A Swiss government agency has also reported that it has seen signs of Petya circulating again.
Others reporting problems include the Ukrainian central bank, the aircraft manufacturer Antonov, and two postal services.
Russian oil producer Rosneft and Danish shipping company Maersk also say they face disruption, including its offices in the UK and Ireland.
“We can confirm that Maersk IT systems are down across multiple sites and business units due to a cyber-attack,” the Copenhagen-headquartered firm said via Twitter.
“We continue to assess the situation. The safety of our employees, our operations and customers’ business is our top priority.”
Spanish media reports that the offices of large multinationals such as food giant Mondelez and legal firm DLA Piper have suffered attacks.
And French construction materials company St Gobain has said that it is also fallen victim.
Some are speculating that it is a similar ransomware assault to the massive global Wannacry virus, which hit in May.
Ukraine seems to have been particularly badly hit.
Reports suggest that the Kiev metro system has stopped accepting payment cards while several chains of petrol stations have suspended operations.
Ukraine’s deputy prime minister has tweeted a picture appearing to show government systems have been affected.
His caption reads: “Ta-daaa! Network is down at the Cabinet of Minister’s secretariat.”