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.
Thousands of Irish hospitals thought to have been hit during the major cyber attack on the NHS were infected by a separate, older virus, it has emerged.
Three Irish hospitals raised concerns they had been affected by the WannaCry ransomware attack which crippled UK health services.
Cyber security experts found that they had another older virus, for which protection was available, Ireland's Health Service Executive (HSE) said.
"In all three cases, the hospital was returned to the health network and continued to deliver patient care with no impact," it said.
The HSE added that new security patches had been added to tens of thousands of computers and all those still affected would be dealt with within 48 hours.
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.