A cyberattack which crippled computers all over the world – including those of Nissan and the NHS – could grow on Monday (15 May), the head of Europol, the European Union's police agency, has said.

Rob Wainwright said that more than 200,000 victims have been affected in over 150 countries in what was the worst hacking incident ever experienced. The attack that began Friday is believed to be the biggest online extortion attack ever recorded, with victims including Britain's hospital network and Germany's national railway.

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It took control of users' computers and demanded payments for them to be unlocked, but is expected to return more ferociously, according to Wainwright.

"At the moment, we are in the face of an escalating threat," he said on ITV. "The numbers are going up. I am worried about how the numbers will continue to grow when people go to work and turn on their machines on Monday morning."

"We are running around 200 global operations against cybercrime each year but we've never seen anything like this.

"The latest count is over 200,000 victims in at least 150 countries. Many of those victims will be businesses, including large corporations. The global reach is unprecedented."

The warning comes as security experts have also warned that another attack could be imminent and could possibly be unstoppable.

The original spread of the WannaCrypt ransomware attack was only halted because an "accidental hero" known as MalwareTech. The 22-year old researcher registered the domain of the malware, which effectively stopped the global propagation of the attacks. Darien Huss , who later collaborated with MalwareTech and realised that the registering of the domain name helped stock the spread of the attacks, warned the possibility of more attacks.

"This is not over," Huss, a researcher at Proofpoint told the Guardian. "The attackers will realise how we stopped it, they'll change the code and then they'll start again. Enable windows update, update and then reboot."

Wainwright said the attack gave businesses and governments a much-needed reminder about the importance of cybersecurity.

"It sends a very clear message I think," he said. "All sectors are vulnerable. All sectors should take absolutely seriously the need to run updated systems and to patch when they can do that."