The 400-page report, drawn up by the team which carried out a mission to the central Chinese city of Wuhan where the virus was first encountered in late 2019, is to be issued on Tuesday after diplomats from WHO's 194 member states are briefed on its findings, Tedros said.
The report cited several reasons for all but dismissing the possibility the virus escaped from a lab in Wuhan, a speculative theory that was suggested and promoted by former U.S. President Donald Trump among others.
The report recommends more testing of blood samples taken and stored before the first outbreak in December, more testing of animals from Southeast Asia, and more in-depth study of mass gatherings that could have aided the spread of the virus.
Want to get the biggest stories from PEOPLE every weekday? .
Tedros said the worldwide experts would hold a press conference on Tuesday to discuss their findings.
Lead expert Peter Ben Embarek revelas preliminary findings to media in February.
The long-awaited study has been repeatedly delayed since it was commissioned past year, prompting doubts about input from the Chinese Communist Party (CCP) and its aim of diverting attention away from the controversial government research laboratory, at one stage implicating the United States military, without evidence, and urging the W.H.O.to launch probes into the virus's origins outside of China.
"It is clear that that the Chinese government has not provided all the data needed and, until they do, firmer conclusions will be hard", he said in a statement.
An AP investigation earlier this year found that China was strongly controlling all coronavirus-related research and banned researchers from speaking to the media.
And predictably, the report has prompted heavy criticism from the United States, with questions once again raised over the access granted to Western experts while in China.
The U.S. government has questioned whether the WHO-convened experts were given enough access to reach a reliable conclusion.
A third hypothesis the experts examined was whether the virus may have been imported to Wuhan in frozen food - a favourite theory in Beijing, which has questioned the initial assumption the virus originated in China.
As Britons dipped back into a pre-pandemic life, several experts turned their attention to the origins report.
The report is based largely on a visit by a World Health Organization team of global experts to Wuhan, the Chinese city where COVID-19 was first detected, from mid-January to mid-February.
For instance, more than 40 years of study has still failed to pinpoint the exact species of bat that are the natural reservoir of Ebola.
The researchers listed four scenarios in order of likelihood. Topping the list was transmission through a second animal, which they said was likely to very likely.
Mark Woolhouse, an epidemiologist at the University of Edinburgh, said even that "very low probability" was an overstatement. "Our hope is that researchers may be able to use this information to develop new strategies to identify, prevent, treat and vaccinate against COVID-19".
"The emergence of a new [disease] is always a sequence of unlikely events", he said. Most experts agree with that.
It said highly similar viruses have been found in pangolins, which are another kind of mammal, but also noted that mink and cats are susceptible to the COVID-19 virus, suggesting they could be carriers, too.
Delays in the publication of the findings, drafted by 17 worldwide experts and 17 Chinese counterparts, had been blamed on coordination and translation issues, even as a diplomatic tug-of-war raged in the background over the report's contents.
It was not clear whether the report might still be changed prior to its release. Director-General Tedros Adhanom Ghebreyesus confirmed that he had received the report. Both refused to be identified because they were not authorized to release it ahead of publication.
The report is expected to be released on Tuesday.
World Health Organization director-general Tedros Adhanom Ghebreysus acknowledged receipt of the report but declined to give details, telling a Geneva news briefing: "All hypotheses are on the table and warrant complete and further studies".
The draft report is inconclusive on whether the outbreak started at a Wuhan seafood market that had one of the earliest clusters of cases in December 2019.