The theory that Covid-19 originated from a Wuhan laboratory leak was discussed by G7 world leaders on Saturday, in a sign that the hypothesis is increasingly being taken seriously.
Dr Tedros Adhanom Ghebreyesus, director general of the World Health Organisation (WHO), confirmed that the theory was raised during a formal session at the G7 summit in Cornwall dedicated to health matters.
He told reporters at a briefing on Saturday that identifying the source of the pandemic was a core part of showing respect to the millions of people around the globe who have died from the virus.
"The first phase of the origin study was not conclusive so there are four hypotheses, but [it’s] not conclusive yet. So we believe that all four hypotheses should be open, and we need to proceed with the second phase, to really know the origins,” he said.
He continued: “As you know, more than 174 million people have been confirmed for Covid illness. This is actually an underestimate, it could be more. And so far, close to 3.75 million people have died. This is very tragic and I think the respect these people deserve is knowing what the origin of this virus is so that we can prevent it from happening again."
Stressing that the WHO "will need cooperation from the Chinese side", he highlighted "difficulties” in "data-sharing", especially the exchange of raw data, during the first phase of the pandemic origin study.
He called for better cooperation and transparency in the second phase of the investigation.
Asked whether the Wuhan laboratory leak theory was discussed by leaders during the health session on Saturday, he said: "Yes, it was raised, and we discussed about the origins."
The hypothesis centres on the idea that the virus could have escaped from a laboratory in the central Chinese city of Wuhan, where cases were first discovered.
Some scientists believe it could have accidentally passed to humans at the Wuhan Institute of Virology, where studies of coronaviruses were taking place.
A more niche version of the hypothesis is that it was engineered.
China’s chief diplomat Yang Jiechi last week said he was "gravely concerned" by the lab leak theory, which he dismissed as an "absurd story".
Beijing "firmly opposes any despicable acts that use the epidemic as an excuse to slander China and to shift blames," Mr Yang was quoted as saying by the official Xinhua News Agency.
The US administration has accused China of withholding raw data and access to sites that would aid deeper investigation into how the virus came into being, and how it first spread.
Earlier this month, Dr Anthony Fauci, the White House chief medical adviser, demanded Beijing release the medical records of several people whose mysterious illnesses could provide crucial insights into the origins of the virus.
He said he was "not convinced" the virus emerged naturally.
Mr Biden has given US intelligence agencies 90 days to step up efforts to study the origins of the virus.
His administration initiated a move for the G7 to call for a fresh investigation in the source of the pandemic.
Many scientists still believe the virus leapt into the human population from an animal via a "natural event" rather than escaping from a laboratory.