The possibility COVID-19 could have leaked from a Chinese lab should not be ruled out, a former top Chinese government scientist has said.

Professor George Gao, the former head of China’s Centre for Disease Control (CDC), told the BBC Radio 4 podcast Fever: The Hunt for COVID’s Origin: “You can always suspect anything. That’s science. Don’t rule out anything.”

This is in contrast to the position of the Chinese government, which dismisses any suggestion the virus may have originated in a Wuhan laboratory.

Wuhan – the city where the virus was first detected – is home to one of China’s top national laboratories known to have spent years studying coronaviruses.

Professor Gao played a key role in the pandemic response and efforts to trace how it started.

The COVID-19 lab leak theory resurfaced earlier this year in February when the FBI’s director said this is where it “most likely” originated.

But many scientists say the weight of evidence suggests that a natural origin – the virus spreading from animals to humans – is the most likely scenario.

In a report seen by the Wall Street Journal, four other US agencies agreed with the FBI and said a lab leak was likely the result of natural transmission.

Another two agencies remain officially undecided, the Journal added.

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Professor Gao told the BBC a formal investigation into the Wuhan Institute of Virology was carried out and “that lab was double-checked by the experts in the field”.

He added: “I think their conclusion is that they are following all the protocols. They haven’t found (any) wrongdoing.”

What has China said about the ‘lab leak’?

“The so-called ‘lab leak’ is a lie created by anti-China forces. It is politically motivated and has no scientific basis,” the Chinese embassy in the UK told the BBC.

Earlier this year, Foreign Ministry spokeswoman Mao Ning said the involvement of the US intelligence community was evidence enough of the “politicisation of origin tracing”.

“By rehashing the lab-leak theory, the US will not succeed in discrediting China, and instead, it will only hurt its own credibility,” Ms Mao said.

“We urge the US to respect science and facts… stop turning origin tracing into something about politics and intelligence, and stop disrupting social solidarity and origins cooperation,” she added.

When did COVID-19 first start spreading?

According to the UK government website, in December 2019, the World Health Organisation (WHO), was informed of a cluster of cases of pneumonia of unknown cause detected in Wuhan City, China.

In January 2020, it was announced that a novel coronavirus (SARS-CoV-2) had been identified from patient samples.

A month later, the virus was formally named SARS-CoV-2, and the disease caused by it was named COVID-19.

According to the WHO, an estimated 336.8 million life-years were lost globally due to the pandemic.