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.
Government cuts are to blame for exposing NHS services to the cyber-attack which hit computers around the world on Friday, Jeremy Corbyn has said.
The Labour leader was speaking as he set out a promise to provide an extra £37 billion for the NHS, with measures aimed at improving A&E performance and taking one million patients off waiting lists.
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.