The NHS was attacked by a cyber-virus on Friday which forced the health service to cancel treatments and appointments.
John Clark, professor of computer and information security at the University of Sheffield, said: “The cyber-attack was very serious indeed. It’s the biggest example of a ransomware attack we have ever seen.
“It could have been prevented by having up to date operating systems.”
It raises the question of why the NHS hadn’t updated their systems.
Mr Clark said: “It is probably funding issues.”