Coronavirus has mutated into two strains, one which appears to be far more aggressive, scientists have said, in a discovery which could hinder attempts to develop a vaccine.
Researchers at Peking University's School of Life Sciences and the Institut Pasteur of Shanghai, discovered the virus has evolved into two major lineages - dubbed 'L' and 'S' types.
The older 'S-type' appears to be milder and less infectious, while the 'L-type' which emerged later, spreads quickly and currently accounts for around 70 per cent of cases.
Genetic analysis of a man in the US who tested positive on January 21, also showed it is possible to be infected with both types.
The finding comes just days after government health experts warned that the virus could hit Britain in ' multiple waves', and led to fears that some vaccines might not work on mutated strains.
Dr Stephen Griffin, of the Leeds Institute of Medical Research and chair of the virus division at the Microbiology Society, said that two of the changes between the 'S' and 'L' lineages were in crucial protein called 'spike', which plays a key role in the infection process and is a target for vaccines.
Dr Griffin said developers would need to test whether their prototype vaccines would still neutralise viruses with the changes, but added that the variations were 'fairly limited' and may not be a 'huge hurdle.'
"It is usually the case that when RNA viruses first cross species barriers into humans they aren't particularly well adapted to their new host - us!" said Dr Griffin.
"Thus, they usually undergo some changes allowing them to adapt and become better able to replicate within, and spread from human-to-human."
Some 35 laboratories and institutions are racing to develop a vaccine with several ready to move to human trials within the next month. But a wide scale inoculation is unlikely to be rolled out before next year.
Virologist Professor Jonathan Ball also warned that mutations could affect vaccine production, but said that the Chinese results needed replication with a larger study.
"At the moment we don't have hard evidence that the virus has changes with regards to disease severity or infectivity so we need to be cautious when interpreting these kinds of computer-based studies, interesting as they might be," he added.
The Chinese scientists, who analysed the viral DNA from 103 infected people, said it appeared the less dangerous 'S-type' was now taking over, possibly because of aggressive public health lockdown measures in China, which had stopped the more virulent disease in its tracks.
However Dr Bharat Pankhania, Senior Clinical Lecturer at the University of Exeter Medical School, said it was impossible to know what was causing the slowdown.
"We don't really know the true picture in China," he said. "Is the lower number of cases a true reflection? It could be the virus mutating to a less dangerous form, or it could be the superhuman lockdown measures.
"Or there could be a third reason which is genetic drift. RNA viruses are quite mistake-prone so over time they become not very good at replicating."
New mutations were also discovered in the case of a 61-year-old man from Brazil, although Dr David Heyman of the London School of Hygiene and Tropical Medicine said a vaccine should still work on the emerging strain.
"Nothing has occurred that is major and this virus appears to be stable," he said.
"Small mutations are normal, especially with RNA viruses. We look for the parts of the virus that are most sustained."
This week the government set out its battle plan to combat the outbreak, warning that in a worst-case scenario, 80 per cent of the population could become infected, with a relatively high death rate among the elderly and frail.
Government scientific experts predict the outbreak could last around four to six months. The government's Chief Medical Officer (CMO) Professor Chris Whitty said he believed there was now onward transmission between people in the UK - which could lead to spiralling numbers - although it is unclear which variant is spreading in Britain.
Prof Witty said Britain was unlikely to follow China's example of shutting down cities, because the virus has already spread too far.
"Closing cities is really only appropriate if you have a significant epidemic in one particular place and almost nothing anywhere else," he said.
"It made sense for China to respond in the way it did but it would be very unlikely here... This is now in multiple places in Europe and around the world."
There are now more than 90,000 cases worldwide with more than 3,000 deaths.
The new findings were published in the National Science Review, the journal of the Chinese Academy of Sciences.