The World Health Organization's Director General has urged further investigation into a theory that Covid-19 sprang from a laboratory leak.

Tedros Adhanom Ghebreyesus, who has been accused of complacency toward Beijing, also rebuked China for sitting on key data.

The theory that the new coronavirus may have escaped from a lab in Wuhan, the Chinese city where it was first detected in humans in December 2019, was a US favourite under former president Donald Trump.

China has always flatly rejected the hypothesis.

And the team of international experts sent to Wuhan by the WHO earlier this year to probe the pandemic's origins have also all but ruled it out.

Their long-delayed report, written alongside the team's Chinese counterparts and published today, ranked four hypotheses in order of probability.

A team of experts from the WHO visited Wuhan in China in January

They said the SARS-CoV-2 virus that causes Covid-19 disease most probably jumped from bats to humans via an intermediary animal, judging a lab leak to be an "extremely unlikely" source.

But Mr Tedros said today that the probe into Wuhan's virology labs had not gone far enough, adding that he was prepared to launch a fresh investigation.

"I do not believe that this assessment was extensive enough," he told the UN health agency's 194 member states, in a briefing on the Covid origins report.

"Further data and studies will be needed to reach more robust conclusions," he said. 

"Although the team has concluded that a laboratory leak is the least likely hypothesis, this requires further investigation, potentially with additional missions involving specialist experts, which I am ready to deploy."

Tedros also urged China to be more forthcoming with data - a call echoed by several countries led by the United States, Britain and Japan.

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Tedros said the Wuhan mission found that the first detected case had symptom onset on 8 December 2019 - but to understand the earliest cases, scientists would benefit from full access to data stretching back to at least September 2019.

The team "expressed the difficulties they encountered in accessing raw data. I expect future collaborative studies to include more timely and comprehensive data sharing," said Tedros.

Hitherto, the WHO chief has faced allegations throughout the pandemic of being too close to China - notably from the former Trump administration, which zeroed in on the lab leak theory.

Peter Ben Embarek, who led the international mission to Wuhan, said the report had "only scratched the surface" in the quest to find the origins of the pandemic, which has killed nearly 2.8 million people and battered the global economy.

The WHO food safety expert said he was "optimistic" the team would discover more "and get closer to the final answer".

Men walk past a sign on Covid-19 in London

On the laboratory accident hypothesis, the Danish scientist said the natural initial reaction to the outbreak of the virus, in a city housing virology labs, was to make the connection between the two.

"Even the staff in these labs told us that was their first reaction," Ben Embarek told journalists.

"They all went back to their records... but nobody could find any trace of something similar to this virus in their records or their samples.

"Nobody has been able to pick up any firm arguments or proof or evidence that any of these labs would have been involved in a lab leak accident."

That said, Ben Embarek added: "we haven't done a full investigation or audit of any of the labs."

He said there had been hours of discussions with laboratory staff and management. The documentation seen by the team had not led them to believe there was anything further left to explore.

But the balance of the hypotheses could be reassessed as further leads come in, he added.

The United States, Japan, Canada, South Korea, Australia, Israel, Britain and seven other European countries issued a joint statement voicing concerns that the investigation "was significantly delayed and lacked access to complete, original data and samples".

Independent experts needed "full access to all pertinent human, animal, and environmental data, research, and personnel", they said.

China's foreign ministry said in a statement: "Actions politicising this search for the origins will only seriously hinder global cooperation in this regard, undermine the global anti-epidemic efforts and cause more loss of life."

The coronavirus has led to nearly 2.8 million deaths worldwide, according to figures from the Johns Hopkins University, while 127 million people have been infected by the virus first detected in China in late 2019.

The letter said Covid-19 has been the "biggest challenge to the global community since the 1940s", noting the two world wars brought about an era of co-operation between nation states.