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.
The Home Secretary has revealed that 48 of the 248 NHS trusts in England have been hit during a "major" cyber attack.
A further 13 NHS Health Boards in Scotland were also targeted in the attack.
Speaking after chairing an emergency Cobra meeting, Amber Rudd added that all but five NHS England trusts are now back to normal, praising the "good work" and "resilience" of NHS staff in making this happen.
Asked if Health Secretary Jeremy Hunt would be making a statement on the attack, the 53-year-old that "plenty of NHS representatives" already had done and were due to.
Amid suggestions outdated software left some health service systems vulnerable after a security package was stopped in 2015, Ms Rudd said it is important to remember that it was not just the NHS which had been affected.
"If you look at who's been impacted by this virus, it's a huge variety across different industries and across international governments.
"This is a virus that attacked Windows platforms. The fact is the NHS has fallen victim to this.
"I don't think it's to do with that preparedness. There's always more we can all do to make sure we're secure against viruses, but I think there have already been good preparations in place by the NHS to make sure they were ready for this sort of attack."
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.