NHS England was the subject of the ransomeware attack, crippling IT and phone systems.
But the Prime Minister said the attack had encompassed a number of different countries.
"This was not targeted at the NHS, it's an international attack and a number of countries and organisations have been affected," May said.
"We're aware that a number of NHS organisations have reported that they've suffered from a ransomware attack," May said. "We're not aware of any evidence that patient data has been compromised."
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