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 aftermath of the "major" cyber attack which has hit the NHS is likely to last for a number of days, experts have said.
A total of 48 NHS trusts in England and 13 NHS health boards in Scotland were crippled in the global attack on Friday when a ransomware virus infiltrated dozens of NHS organisations.
Five NHS England trusts were still not back to normal on Saturday, despite NHS Digital engineers working "around the clock" in a bid to fix the problem.
NHS Digital continued that fewer than five per cent of devices within the health service still use the old Windows XP system.
"We are aware of widespread speculation about the use of Microsoft Windows XP by NHS organisations, who commission IT systems locally depending on population need.
"While the vast majority are running contemporary systems, we can confirm that the number of devices within the NHS that reportedly use XP has fallen to 4.7%, with this figure continuing to decrease.
"This may be because some expensive hardware (such as MRI scanners) cannot be updated immediately, and in such instances organisations will take steps to mitigate any risk, such as by isolating the device from the main network."
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.