The inquiry into baby deaths found the Countess of Chester Hospital, in northwestern England, could have saved some infants from death and injury if staff had acted sooner when children began collapsing, Justice Kathryn Thirlwall said on Tuesday.
"Errors were made by nurses, doctors and managers," Thirlwall said.
"There was also complete failure at all levels to invoke safeguarding procedures at any point."
Letby, 36, is serving a life sentence for the murders of seven babies and attempted killing of seven others.
She has maintained her innocence, and a defence team backed by dozens of scientists who have questioned the evidence against her are seeking to clear her name.
Thirlwall's inquiry did not review Letby's convictions but looked at how institutional failures exposed babies to repeatedly be harmed at the hospital and how staff and management responded and treated parents.
She focused on the failure of staff to intervene when it became clear someone was deliberately harming babies.
"How many lives could have been saved had the hospital acted differently?" she said.
"It is clear that some babies would have been saved and some attacks would have been prevented if action was taken earlier."
Letby was convicted in 2023 of the murders of seven infants and attempted murders of six others - including two attempts on one child.
A case in which jurors could not reach a decision was retried and Letby was convicted in July of another attempted murder.
She was sentenced to 15 life terms with no chance of release - only the fourth woman in the United Kingdom to receive such a term.
Prosecutors said she harmed babies in ways that left little trace, including injecting air into their bloodstreams, administering air or milk into their stomachs via nasogastric tubes, poisoning them with insulin and interfering with breathing tubes.
She was the only employee on duty in the neonatal unit when the children collapsed or died between June 2015 and June 2016.
Prosecutors described her as a "constant malevolent presence".
