The possibility of a lab leak as the source of the pandemic was "extremely unlikely", according to a draft copy of the findings obtained by The Associated Press.
Former US president Donald Trump previously claimed he'd seen proof the coronavirus outbreak originated from an infectious disease laboratory in Wuhan.
The findings were largely as expected and left many questions unanswered, with the team proposing further research in every area except the lab leak hypothesis.
The release of the report has been repeatedly delayed, raising questions about whether the Chinese side was trying to skew the conclusions to prevent blame for the pandemic falling on China.
The AP received what appeared to be a near-final version on Monday from a Geneva-based diplomat from a WHO-member country.
It was not clear whether the report might still be changed prior to its release.
The diplomat did not want to be identified because they were not authorised to release it ahead of publication.
The researchers listed four scenarios in order of likelihood.
They concluded that transmission through a second animal was likely to very likely.
They evaluated direct spread from bats to humans as likely, and said that spread through “cold-chain” food products was possible but not likely.
The closest relative of the virus that causes Covid-19 has been found in bats, which are known to carry coronaviruses - but the report said “the evolutionary distance between these bat viruses and SARS-CoV-2 is estimated to be several decades, suggesting a missing link”.
It said that highly similar viruses have been found in pangolins, but also noted that mink and cats are susceptible to the Covid virus, which suggests they could be carriers.
The report is based largely on a visit by a WHO team of international experts to Wuhan, the Chinese city where Covid-19 was first detected, from mid-January to mid-February.
Peter Ben Embarek, the WHO expert who led the Wuhan mission, said on Friday that the report had been finalised and was being fact-checked and translated.
“I expect that in the next few days, that whole process will be completed and we will be able to release it publicly,” he said.