Experiments with a mutant Covid strain are said to have been carried out by Chinese scientists, despite concerns the research could spark another pandemic.
The mutant strain is thought to be 100 per cent lethal in mice and spreads through the body in a "unique way".
The Covid-like virus was cloned by scientists in Beijing, who are linked to the Chinese military, and used it to infect mice.
It has been claimed that the experiment saw the mice engineered to express a protein found in people, in a bid to see how the virus might react in humans.
Experiments with a mutant Covid strain are said to have been carried out by Chinese scientists, despite concerns the research could spark another pandemicGettyResearchers were surprised to see how the virus multiplies and spreads after finding high levels of viral load in the mice's brains and eyes.
An unpublished paper warned the finding "underscores a spillover risk of GX_P2V into humans".
Professor Francois Balloux, an infectious disease expert based at University College London, wrote on social media: "It's a terrible study, scientifically totally pointless.
"I can see nothing of vague interest that could be learned from force-infecting a weird breed of humanized mice with a random virus. Conversely, I could see how such stuff might go wrong..."
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The virus was reportedly discovered in 2017, before the Covid outbreak.
According to the study, it was discovered in Malaysia in pangolins - mammals which commonly harbour coronaviruses and were suspected to be the intermediate host that passed Covid from bats to humans.
"The preprint does not specify the biosafety level and biosafety precautions used for the research," Professor Richard Ebright, a chemist at Rutgers University in New Brunswick, New Jersey, told the Daily Mail.
"The absence of this information raises the concerning possibility that part or all of this research, like the research in Wuhan in 2016-2019 that likely caused the Covid-19 pandemic, recklessly was performed without the minimal biosafety containment and practices essential for research with a potential pandemic pathogens.
The virus was reportedly discovered in 2017, before the Covid outbreak
PexelsIt is unknown when the study was conducted, but researchers said it was possible the virus had undergone a "virulence-enhancing mutation" in storage - which resulted in it becoming more deadly.
Researchers suggest that the new strain infects the respiratory system and then migrates to the brain - unlike Covid which causes lower lung infections and pneumonia in severe cases.
They concluded: "Severe brain infection during the later stages of infection may be the key cause of death in these mice."