Health trusts across England were sent details of an IT security patch that would have protected them from the crippling ransomware attack, NHS Digital said.
Large swathes of the NHS have been paralysed by the cyber attack, which hit 200,000 victims in 150 countries around the world.
But NHS Digital said it had made health trusts aware last month of IT protection that could have prevented the attack.
Theresa May has rejected claims the government ignored warnings the NHS was vulnerable to a possible cyber security attack.
The Prime Minister said warnings had been given to hospital trusts. During a visit to Oxfordshire, she insisted cyber security was being taken seriously in Whitehall.
Asked if warnings had been ignored, Mrs May said: "No. It was clear warnings were given to hospital trusts but this is not something that focused on attacking the NHS here in the UK."
She added: "Europol say there are 200,000 victims across the world.
"Cyber security is an issue that we need to address. That's why the Government, when we came into Government in 2010, put money into cyber security."
Hospitals have stopped diverting patients following last week's cyber attack which crippled many NHS Trusts' IT systems.
A young computer expert from north Devon who was hailed a hero for helping to stop the global cyber attack said he was just "doing my bit".
One surgery has no access to medical records.