Finding the origins of a virus is “notoriously difficult”, an expert has said.

Dr David Nabarro, World Health Organisation (WHO) special envoy on Covid-19, said that “it takes longer than people think” to discover where a disease has come from.

The precise origins of HIV and Ebola have not yet been identified, he added.

It comes as the WHO plans to release its report on its exploratory visit to Wuhan to investigate the origins of the virus.

Dr Nabarro told BBC Radio 4’s Today programme: “Finding the origins of a  virus, when you’re trying to explain where a disease has come from, is notoriously difficult.

“We don’t know the precise origins of HIV, we don’t know the precise origins of Ebola, and it will take a long time to find the precise origins of Covid-19.

“We’re on the way – we’ve got four hypotheses, we’re testing them all.

“The research that needs to be done is known.”

He added: “It always takes much longer than people think to find where these things have come from.”

Meanwhile, epidemiologist Dr Mike Tildesley, who advises the Government on the Scientific Pandemic Influenza Group on Modelling ( SPI-M), told BBC Breakfast the results of the WHO report into the origins of Covid-19 could help guard against future pandemics.

He said: “We do know that often a lot of the viruses that affect us do come from bats, sometimes we do have these intermediary hosts where it’s easier to transmit to humans.

“We saw a similar thing 10 years ago with swine flu, which probably pales into insignificance now with Covid, but we had pigs as an intermediary host.

“It’s the sort of thing we are looking for and helps us to inform future surveillance procedures to make sure we are alert for any possible evidence of zoonotic transmission in the future, so we can ensure we react and hopefully avoid another large-scale pandemic happening in the future.”

The WHO has previously outlined initial findings after a visit to Wuhan in China.

Officials said the virus which causes Covid-19 is “extremely unlikely” to have entered the human population as a result of a laboratory-related incident.

Sars-CoV-2 “may have originated from zoonotic transmission”, meaning it passed from animals to humans, they concluded.

Experts said the early findings indicate that the virus was introduced to humans through an “intermediary host”, which means it jumps from one species to another, and then jumps from the second species to humans.

But the “reservoir hosts” – the animals in which the virus originated – remain to be identified, they told a press conference.

The first probable case of the virus investigated by the team was on December 8 2019.

This case did not have any links to the animal market, Wuhan’s Huanan Seafood Wholesale Market, which was initially thought to be where the virus “spillover” from animals to humans occurred.

China has faced claims that the Wuhan Institute of Virology could be the suspected source of the Covid-19 virus.

But the team of experts from WHO and China ruled out the possibility.

Investigators looked at four hypotheses when examining the possible origins of the virus:

– Direct transmission from an animal species into the human population.

– The introduction of the virus from an intermediary host species through another animal species “potentially closer to humans” where the virus could adapt, circulate and then jump to humans.

– The food chain, in particular the potential for frozen products acting as a “surface” for the transmission of the virus.

– A lab-related incident.

They ruled out a possible lab incident and set out further research which needed to be conducted in the other three areas.

Since the virus was first detected in December 2019, there have been more than 127 million cases around the globe and 2.8 million deaths.