'It's an worldwide attack and a number of countries and organisations have been affected, ' British Prime Minister Theresa May said. "Maximum vigilance needed in days ahead".
He said IT experts had been working through the weekend to get the system back up and running again and were still working on the main server first thing on Monday morning.
"We are in the second wave", said Matthieu Suiche of the cybersecurity firm Comae Technologies on Sunday, per The New York Times. It is not clear why it took so long for NHS Digital to send it, or why so many trusts failed to update their systems once notified.
"We are planning to continue as we have today but can not confirm when our computers will be back working".
While some practices were not directly attacked by the cyber-attack, some network connections to data servers and the Internet were affected.
"It's why we are putting £2 billion [$2.6 billion] into cyber-security over the coming years and, of course, created the National Cyber Security Centre".
Organisations across the globe, including investigators from the UK's National Crime Agency, are now working non-stop to hunt down those responsible for the ransomware.
They proposed a plan to improve security that included a replacement of outdated systems "as a matter of urgency", calling the continued use "one of the most pressing issues facing IT infrastructure" in the NHS.
Whiteman said priorities about where to spend resources and invest in the workforce should be made in the context of risk management strategies, which assess both the likelihood of problems occurring and the impact if they do.
Experts say another attack could be imminent and have warned people to ensure their security is up to date.
But he warned it was likely some Australians would fall victim.
Hunt told broadcaster Sky News, the United Kingdom had "never seen a ransomware attack on this scale".
"And also existing known infections can spread".
Companies were being encouraged to update their Microsoft systems so the software isn't vulnerable to malware like WannaCry, which puts a lock on computers, steals its data, and then demands payment in Bitcoins before later destroying the data it collected.
"If you have a scheduled appointment you are advised to attend as normal unless you are told otherwise".