A Chinese scientist may have started the pandemic after being infected with coronavirus while collecting bat samples, the head of the World Health Organisation's investigation has said.
In a documentary released this week by the Danish television channel TV2, Dr Peter Embarek said it was a "likely hypothesis" that a lab employee could have picked up the virus while working in the field.
Scientists from the Wuhan Institute of Virology were known to be working on bat coronavirus at labs in the city, but China has been uncooperative in providing details of their research.
Embarek said WHO investigators were forced to conclude that a lab leak was "extremely unlikely" in their official report to avoid further arguments with the Chinese.
He said the team had come to an "impasse" with China, which would only allow a lab leak scenario to be included in the report if there were no recommendations to look further.
"My counterpart agreed we could mention [the lab leak scenario] in the report under the condition that we wouldn't recommend specific studies of that hypothesis. We would just leave it there."
Asked whether the Chinese would have agreed to the report without the scenario being labelled "extremely unlikely", Dr Embarek said: "That would have probably demanded further discussion and arguments for and against I didn't think it was worth it."
However, Embarek said it was possible that a lab employee may have been infected in the field.
"We consider that hypothesis a likely one," he added.
Peter Ben Embarek, Danish researcher on WHO-led probe into covid origins, says Chinese colleagues blocked lab-leak theory - The Washington Post https://t.co/gEaIc6QhOu
Pressure is growing on China to release documentation of work at laboratories in Wuhan and allow a thorough investigation.
A report into the lab leak scenario, which was commissioned by US President Joe Biden, is expected to report at the end of August, and last month the WHO called for an in-depth audit, a request that the Chinese had rejected.
Sir Iain Duncan Smith, co-chairman of Britain's Inter-Parliamentary Alliance on China, said the international community urgently needed to identify how the virus outbreak erupted.
"There's no question now that this process needs to be undertaken by the WHO. They need to come clean, as China needs to come clean, about the origins of the virus," he said.
'Arrogant refusal to accept virus origins'
Sir Iain said millions of people had lost their lives on account of the "terrible and arrogant refusal to accept that the origins of the virus" may be linked to the Wuhan lab.
Dr Embarek also told the documentary team that he was concerned about a second lab, the Wuhan Centre for Disease Control and Prevention (CDC), which had moved premises to just a third of a mile from the Wuhan wet market where the outbreak first emerged.
"There are other labs in Wuhan that are interesting, such as CDC, which also worked with bats," he said.
"What is more concerning to me is the other lab that is next to the market, because they were also handling coronaviruses, without potentially having the same level of expertise or safety ...
"When we were being shown around I thought it all looked new. I asked how old the lab was and they said, 'We moved on December 2'.
"That's when it all started. We know that when you move a lab it disturbs all the procedures. You have to move the virus collection and the samples. That's why that period of time and that lab are interesting."
Lab leak theory persists
Experts in Britain said it was "plausible" that a lab employee could have brought the virus back to Wuhan, which would also fit with genetic studies showing it had jumped from an animal.
Dr Jonathan Stoye, group leader of the Retrovirus-Host Interactions Laboratory at The Francis Crick Institute, said: "It sounds entirely plausible to me.
"My feeling when I read the original WHO report was there was no grounds for calling it extremely unlikely so it was always slightly strange.
"I have been saying for a while that this isn't solved, the lab link is still there and we need to know more. The question is how we go about getting more.
"To my mind, there is no evidence of manipulation of the virus, but we know these investigators have been collecting bat samples, so they could have carried something back."
Genetic studies support lab leak scenario and wild infection
Ravi Gupta, professor of microbiology at the University of Cambridge, said that current genetic studies supported both a lab leak scenario and a wild infection.
"The genetics are consistent with the lab leak/field work infection scenario described by the WHO mission lead, and also consistent with infection from the wild in general by a non-lab worker," he said.
However, other researchers said the comments did little to move the investigation forward.
"There are many possible ways the virus was transmitted to humans," said Professor David Robertson, head of viral genomics and bioinformatics at the University of Glasgow.
"Peter was just referring to something that was possible. As we've no evidence for this, or any link to a lab leak, it remains just speculation."