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.
Two hospitals remain on "divert" status as the NHS recovers from a cyber attack which has affected more than 40 Trusts.
NHS England announced on Monday that the number of hospitals sending patients to other facilities was down from seven to two.
Lister Hospital in Hertfordshire and Broomfield Hospital in Essex are still re-directing people seeking medical treatment following the ransomware attack, which affected organisations across 150 countries.
Dr Anne Rainsberry, from NHS England, said: "There are encouraging signs that the situation is improving, with fewer hospitals having to divert patients from their A&E units.
“The message to patients is clear: the NHS is open for business. Staff are working hard to ensure that the small number of organisations still affected return to normal shortly."
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.