"This may be the last opportunity to ensure the WHO is empowered" to build an effective emergency response capacity, warned an advance, unedited copy of the UN panel's report, made available online over the weekend in a link on the United Nations' daily Journal website.
"If the WHO does not successfully reform, the next major pandemic will cause thousands of otherwise preventable deaths."
The UN report, entitled "Protecting Humanity from Future Health Crises", is the latest in a series of reviews by global health experts which have been sharply critical of the WHO's response to the devastating Ebola epidemic in West Africa.
- WHO's delay in sounding Ebola alarm 'egregious failure,' expert panel says
- Post-Ebola Syndrome: Survivors continue to face mystery symptoms
With all eyes now focused on the response to the mosquito-borne Zika virus, which has been reported in 33 countries and is linked to birth defects, the agency is under even more scrutiny. WHO chiefs have previously promised to act swiftly on reforming the agency's emergency responses.
It was not immediately clear when the final report will be published.
No substitute for single global health leader
The UN panel's key recommendation was for the WHO to build a new Centre for Emergency Preparedness and Response, which "must have real command and control capability, access to specialised human and operational resources to execute a health response."
Jeremy Farrar, director of the Wellcome Trust global health charity, agreed with the panel's recommendation for the creation of a new centre, which he said must be "strong and independent."
"After four inquiries into the preventable tragedy of Ebola, there is now a strong consensus about what must be done.
The WHO's leadership and member states must make 2016 the year of decision and act now."
But failure by the Geneva-based WHO to reform, it said, "may necessitate consideration of alternate UN institutional response mechanisms."
"Had they been implemented, thousands of lives could have been saved in West Africa," it said.