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.
Health Secretary Jeremy Hunt has said there has not been a second wave of cyber attacks after the NHS was struck by ransomware attacks on Friday.
Mr Hunt said all organisations need to do more to protect themselves from cyber attacks, which he said were "relatively common".
He said: "Although we have never seen anything on this scale when it comes to ransomware attacks, they are relatively common and there are things that you can do, that everyone can do, all of us can do, to protect ourselves against them.
"In particular, making sure that our data is properly backed up and making sure that we are using the software patches, the anti-virus patches that are sent out regularly by manufacturers.
"These are things that we can all do to reduce the risk of the impact that we've seen over the last 48 hours."
Mr Hunt has come under fire for failing to appear in public since the attack, which hit 47 trusts in England and 13 Scottish health boards on Friday.
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.