Wuhan scientists were studying viral samples of high-risk bat species living in Laos - the country where the closest relative to Covid-19 has been found, leaked documents show.

In September, researchers discovered a viral strain called Banal-52 in Laos which shares 96.8 per cent of its genome with Sars-CoV-2, bolstering claims that the pandemic was caused by a natural spillover event, rather than a lab leak.

However it remained unclear how a bat-borne virus from Laos could have ended up sparking an outbreak in Wuhan, more than 1,000 miles away.

Now, leaked emails between virus hunters EcoHealth Alliance and US government funders, show viral samples were being collected from bats in Laos and sent back to the Wuhan Institute of Virology for study.

The emails, uncovered by the US-based White Coat Waste Project, suggest that viral DNA from "bats and other high-risk species" were sent to Wuhan between June 2017 to May 2019.

Before Banal-52 was found, the closest relative to Covid-19 was discovered in cave bats in Yunnan, China, where EcoHealth Alliance had also been looking for viruses and sending samples back to the Wuhan Institute of Virology.

The genetic sequences collected from both Yunnan and Laos were kept in an online database at the lab, but that was removed in September 2019, leaving experts unsure of exactly what strains were studied there.

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Viscount Ridley, co-author of Viral: The Search for the Origin of Covid-19, said the emails added more weight to the theory that the virus leaked from the Wuhan lab.The peer said: "Banal-52 is not close enough to be the progenitor, it's still not the smoking gun, but it's pretty good. So maybe this virus started in Laos, not China. Interesting possibility.

"But we got a leak of a document showing that the EcoHealth Alliance was sampling bats in Laos."

They say in the document that because it would be complicated to come back and ask the US government for permission to give some of the grant to a Laotian lab, they'd like to send all these samples to a lab that can analyse it for them. It was in a place called Wuhan.

"So the outbreak happened in a city with the world's largest research programme on bat-borne coronaviruses, whose scientists had gone to at least two places where these Sars-CoV-2-like viruses live."

Gilles Demaneuf, a researcher and member of Drastic, the web pandemic origins group, wrote in a recent blog: "Now we have a very plausible direct route from Laos to Wuhan, a route with two options. Number one, a Wuhan bat sampler infected on a field sampling trip. Number two, a research accident in Wuhan when manipulating a Laos Banal-like bat coronavirus."

He points out that such manipulation of viruses had already been discussed in a grant application by EcoHealth Alliance, who were collaborating with the Wuhan lab.

Proposals to the US Defence Advanced Research Projects Agency, leaked to Drastic earlier this year, show the scientists were hoping to introduce "human-specific cleavage sites" to bat coronaviruses which would make it easier for the virus to enter human cells.

The team also planned to take sequences from naturally-occurring coronaviruses and use them to create a new sequence that was an average of all the strains. Genetics experts have said that if Covid-19 had been produced in this way, it would explain why a close match has never been found in nature.