The theory that coronavirus was released accidentally from a laboratory in the Chinese city of Wuhan “remains viable” and must be investigated further, a group of senior scientists has said.

An investigation into the origin of the pandemic carried out last year by the World Health Organization and facilitated by China found that human infection by an intermediate animal host was a “likely to very likely” explanation.

It rejected the idea that the Sars-Cov-2 virus escaped from a lab such as the Wuhan Institute of Virology, saying it was “extremely unlikely”.

But the WHO probe failed to examine properly the possibility that the virus emerged from a lab, the 18 scientists wrote in a letter published in the journal Science on Friday.

Only four of the 313 pages of the report and its annexes addressed the lab leak theory, which WHO director-general Tedros Ghebreyesus Ghebreyesus subsequently admitted was insufficient.

“Theories of accidental release from a lab and zoonotic spillover both remain viable,” wrote the scientists who work at universities in the US, UK, Canada and Switzerland, adding that “greater clarity about the origins of this pandemic is necessary and feasible to achieve”.

Any future investigation should be transparent, objective and “subject to independent oversight . . . to minimise the impact of conflicts of interest”, they said.

“Knowing how Covid-19 emerged is critical for informing global strategies to mitigate the risk of future outbreaks,” the letter added.

The comments mark the latest twist in the debate over how Covid jumped to humans. As China struggled to control the spread of the virus early last year, a group of international scientists attacked those espousing the lab-leak hypothesis, claiming efforts to share data on the outbreak were being “threatened by rumours and misinformation around its origins”.

In a letter published in the Lancet in February last year, the scientists condemned “conspiracy theories” suggesting Covid did not originate naturally. Speculation did nothing but “create fear, rumours, and prejudice that jeopardise our global collaboration in the fight against this virus”, they said.

China has always rejected claims that the virus accidentally leaked from a medical research facility. They also strenuously deny that Sars-Cov-2 is a genetically engineered coronavirus created at the Wuhan lab as part of “gain of function” experiments.

When the pandemic took hold, the theory that the virus had been passed naturally from an animal to a human host was adopted as the most likely scenario. This was partly down to an absence of proof to the contrary and because previous coronavirus outbreaks such as Sars in the early 2000s was caused by the virus spreading from animals to humans.

Yet it has never been established whether the virus moved to humans directly from bats or via an intermediate animal host such as a pangolin. Attempts to identify the intermediary have been unsuccessful.

Some, such as then US president Donald Trump remained sceptical. Those doubts were given credence when it emerged that one of the authors of the February letter dismissing the lab leak theory held a senior position at a non-profit group that had provided funding for research into coronaviruses at the Wuhan Institute of Virology. The letter had declared no competing interests.

Peter Hale, executive director of the Foundation for Vaccine Research in Washington, said there was “growing support at the highest level of the Biden administration for the notion that the Covid-19 pandemic may well have stemmed from a lab escape”.

He added: “This is not China-bashing as we witnessed during the Trump administration.”