Coronavirus could have spread among humans 'for years or even decades' before it was detected in Wuhan, a team of scientists have claimed.
They believe a strain of coronavirus may have jumped from animal to human as long as a decade ago - and before it was capable of causing the disease in people.
The findings have been published the scientific journal Nature Medicine after a team of experts analysed the pandemic sweeping the globe.
Dr Francis Collins, director of the US National Institute of Health, said "...as a result of gradual evolutionary changes over years or perhaps decades. The virus eventually gained the ability to spread from human to human and cause serious, often life-threatening disease.”
The study adds: "As many early cases of Covid-19 were linked to the Huanan market in Wuhan, it is possible that an animal source was present at this location."
They found bats and pangolins contain coronaviruses similar to Covid-19.
The theory the virus was spreading among people before December has been suggested by a number of other experts.
Italian researchers are looking at whether a higher than usual number of cases of severe pneumonia and flu in Lombardy in the last quarter of 2019 may be a signal that the new coronavirus might have spread beyond China earlier than previously thought.
Adriano Decarli, an epidemiologist and medical statistics professor at the University of Milan, said there had been a "significant" increase in the number of people hospitalised for pneumonia and flu in the areas of Milan and Lodi between October and December last year.
He told Reuters he could not give exact figures but "hundreds" more people than usual had been taken to hospital in the last three months of 2019 in those areas - two of Lombardy's worst hit cities - with pneumonia and flu-like symptoms, and some of those had died.
Decarli is reviewing the hospital records and other clinical details of those cases, including people who later died at home, to try to understand whether the new coronavirus epidemic had already spread to Italy back then.
"We want to know if the virus was already here in Italy at the end of 2019, and - if yes - why it remained undetected for a relatively long period so that we could have a clearer picture in case we have to face a second wave of the epidemic," he said.
The World Health Organization has said the new coronavirus and COVID-19, the respiratory disease it causes, were unknown before the outbreak was first reported in Wuhan, in central China, in December.
Decarli said once his research was concluded, local health authorities might decide to request authorisation to exhume bodies of people with suspect symptoms.
Other experts cast doubt on the hypothesis that the new virus could have been circulating in Europe before the end of 2019.
"I think it extremely unlikely that the virus was present in Europe before January," said Paul Hunter, a professor in medicine at Britain's University of East Anglia who has been tracking the evolving pandemic.
Hunter said that unless Italian scientists get positive results from samples taken and stored at that time, then the suggestion should not be given credence.
He added that, given what we know about how infectious the virus is, and the ratio of patients showing no symptoms compared with those that get sick, "it is inconceivable that we would not have had a pretty major epidemic in Europe much earlier if these cases had in fact been COVID-19".