Security wonks are calling it the biggest cyberattack ever.
Barts Health, which runs five hospitals in London, said Sunday it was still experiencing disruption to its computer systems and it asked for the public to use other NHS services wherever possible.
He warned that more people may be hit by the virus today when they return to work and switch on their computers.
"We are running around 200 global operations against cybercrime each year but we've never seen anything like this", he told Britain's ITV television. As per reports, the ransomware has hit as many as 200,000 victims in over 150 countries.
An Garda Síochána said early Sunday there had been no identified cyber attack on any Irish state computer system from the wave of worldwide ransomware attacks, although RTÉ reported a suspected attack on healthcare facility in the south-east.
"We will get a decryption tool eventually, but for the moment, it's still a live threat and we're still in disaster recovery mode", Rob Wainwright, the head of the European Union's law enforcement agency Europol, told CNN's Becky Anderson on Sunday.
Nevertheless, MalwareTech's purported accidental rescue of more than 100,000 computers in 104 countries was confirmed by the British government's National Cyber Security Centre.
Backup your important data in more than one place, such as an external hard drive. Payment is demanded within three days or the price is doubled, and if none is received within seven days the locked files will be deleted, according to the screen message.
"Remarkably few payments" had so far been made in response to this attack, he added.
Wainwright called the fast-spreading hack "unique" because the ransomware was being used in combination with a worm, meaning that the infection of one computer could automatically spread it through an entire network.
"We are working with customers to provide additional assistance as this situation evolves", the company said.
"It only guarantees that the malicious actors receive the victim's money, and in some cases, their banking information".
Chinese media reported Sunday that the "ransomware" virus attacked many university networks in China.
It is the largest ransomware attack observed in history.
"You never want to pay a criminal as there is no honour amongst thieves but ultimately its going to be a business decision if they think they can not operate without these files", he said.
WannaCry uses a vulnerability in old versions of Windows that was originally discovered and exploited by the U.S. National Security Agency as an offensive cyber-weapon.
The National Cyber Security Centre said: "That, as a new working week begins it is likely, in the United Kingdom and elsewhere that further cases of ransomware may come to light, possibly at a significant scale".
"We have not yet seen Friday's attack reoccur, there's been no new wave of attacks".
Among those affected by the virus was Nissan, but the auto manufacturer said there had been no major impact.
As a result, hospitals, businesses, and governments were affected.
The ransomware started in Russian Federation and quickly infected computer systems ranging from FedEx to the U.K.'s National Health Service, shutting down hospitals, and was only stopped by luck when a 22-year-old British cybersecurity researcher accidentally discovered a "kill switch" hidden in the code, according to the Guardian.