The NHS, Fedex and the main telecoms operator in Spain were among 200,000 known victims of Friday’s global cyber-attack.
But Europol head Rob Wainwright said he was concerned that the number affected would continue to rise when people returned to work on Monday morning.
He told the BBC there was an escalating threat from the virus, known as Wanna Decryptor or WannaCry, adding: “We’ve never seen anything like this – it’s unprecedented in scale.”
The ransomware, which locked users’ files and demanded payment to allow access, spread to 150 countries, including Spain, Russia, the US and China.
In England, 48 trusts reported problems at hospitals, GP surgeries or pharmacies and 13 NHS organisations in Scotland were also affected.
Some hospitals were forced to cancel treatment and appointments and, unable to use computers, many doctors resorted to using pen and paper.
‘Warned again and again’
Asked by Andrew Marr if the government had failed to give the NHS proper support and failed to pay for “crucial” upgrades to security in 2015, Defence Secretary Sir Michael said £1.9bn had been set aside for UK cyber-protection – when cyber-attacks were identified as one of three main threats to the UK’s defences.
Of that, he said: “We’re spending around £50m on the NHS cyber systems to improve their security. We have encouraged NHS trusts to reduce their exposure to the weakest system, the Windows XP.”
Fewer than 5% of the trusts used XP now, he said.
“We want them to use modern systems that are better protected.
“We warned them, and they were warned again in the spring. They were warned again of the threats.”
However, Kingsley Manning, a former chairman of NHS Digital, – which provides the health service’s IT systems – told the BBC on Saturday that several hundred thousand computers were still running on Windows XP.
And shadow health secretary Jonathan Ashworth wrote to Health Secretary Jeremy Hunt on Saturday asking why concerns repeatedly flagged up about the NHS’s “outdated, unsupported and vulnerable” machines had not been addressed.
Speaking to ITV’s Robert Peston, Mr Ashworth accused the government of having “cut the IT and infrastructure budget” by £1bn in the NHS, and said his party, if elected to power, would put £10bn into the infrastructure of the NHS.
Mr Hunt has not publicly responded to the attack, but Home Secretary Amber Rudd said the priority was to “disrupt” it.
Meanwhile, digital rights campaigners Open Rights Group has accused GCHQ of a “very dangerous strategy of hoarding knowledge of security problems”.
It said Britain’s electronic surveillance agency was “in charge of hacking us and protecting us from hackers”, making it hard to balance the risks of keeping vulnerabilities secret.
Jim Killock, the group’s executive director, said: “US and UK security agencies kept a widespread vulnerability secret rather than telling the companies so they could fix it.” He called for the National Cyber Security Centre to be made independent from GCHQ.