More than 10,000 people have been killed and nearly 25,000 infected since the Ebola epidemic was first identified in west Africa in March 2014, mostly in Guinea, Liberia and Sierra Leone.
But Doctors Without Borders - known best by its French initials MSF - said "months were wasted and lives were lost" because the UN's World Health Organization, which is charged with leading on global health emergencies,and "possesses the know-how to bring Ebola under control," failed to respond quickly or adequately.
The report accused the WHO's Global Alert and Outbreak Response Network of ignoring desperate pleas for help from Liberia when it met in June.
"I remember emphasising that we had the chance to halt the epidemic in Liberia if help was sent now," said Marie-Christine Ferir, MSF emergency coordinator.
"It was early in the outbreak and there was still time. The call for help was heard but no action was taken."
The WHO did not set up a regional hub for coordinating the response until July, by which time a second wave of the epidemic had struck.
"All the elements that led to the outbreak's resurgence in June were also present in March, but the analysis, recognition and willingness to assume responsibility to respond robustly were not," the report said.
Particularly in the early months, much of the burden fell on MSF's shoulders, but the organisation had only 40 staff with Ebola experience.
"We couldn't be everywhere at once, nor should it be our role to single-handedly respond," said Brice de le Vingne, MSF director of operations.
It was only when a US doctor and Spanish nurse were diagnosed with Ebola that the world woke up to the threat, MSF said.
The WHO has acknowledged its response was slow and insufficient, but said "from the start all of the levels of WHO... have been mobilised and very deeply engaged in the response."
It said an independent review is underway and will report back in May, and that the organisation was planning to set up a rapid response team "to deal with any outbreak at a moment's notice."
MSF also blamed the governments of Guinea and Sierra Leone for refusing to admit to the scale of the epidemic and putting "needless obstacles" in the path of MSF teams.
The crisis sparked the biggest training programme in MSF history, which deployed 1,300 international staff and 4,000 locals.
It was initially focused on Guinea and Sierra Leone. However, when aid agency Samaritan's Purse withdrew from Liberia after the US doctor's infection, MSF faced a wrenching decision: abandon the country or take the risk of pushing its over-stretched staff still further.
"We couldn't let (Liberia) sink further into hell," said de le Vingne. "We would have to push beyond our threshold of risk, and we would have to send coordinators without experience in Ebola, with only two days of intensive training."
MSF built a 250-bed centre in Liberia's capital Monrovia, but even that was far from enough. The centre able to open only 30 minutes each morning, filling beds vacated by deaths overnight.
The report describes people dying on the gravel outside the gates, and a father who brought his daughter in the boot of his car, begging MSF to take her in so as to not infect his other children at home, but who was turned away.
"It was an indescribable horror," said Rosa Crestani, Ebola task force coordinator.
There were so many patients and so few employees that the staff had on average only one minute per patient.
"This Ebola outbreak has wrought an exceptionally heavy toll on MSF's staff, and particularly on our west African colleagues," the report said.
"Not since the early days of HIV care have MSF staff sustained the loss of so many patients dying in our facilities and never in such an intense short period of time," the report concluded.