The number of people already infected by the mystery virus emerging in China is far greater than official figures suggest, scientists have told the BBC.
There have been more than 60 confirmed cases of the new coronavirus, but UK experts estimate a figure nearer 1,700.
Two people are known to have died from the respiratory illness, which appeared in Wuhan city in December.
"I am substantially more concerned than I was a week ago," disease outbreak scientist Prof Neil Ferguson, said.
The work was conducted by the MRC Centre for Global Infectious Disease Analysis at Imperial College London, which advises bodies including the UK government and the World Health Organization (WHO).
Singapore and Hong Kong have been screening air passengers from Wuhan, and US authorities announced similar measures starting on Friday at three major airports in San Francisco, Los Angeles and New York.
How were the numbers calculated?
The crucial clue to the scale of the problem lies in the cases being detected in other countries.
While the outbreak is centred on the central Chinese city of Wuhan, there have been two cases in Thailand and one in Japan.
"That caused me to worry," said Prof Ferguson.
He added: "For Wuhan to have exported three cases to other countries would imply there would have to be many more cases than have been reported."
It is impossible to get the precise number, but outbreak modelling, which is based on the virus, the local population and flight data, can give an idea.
Wuhan International Airport serves a population of 19 million people, but only 3,400 a day travel internationally.
The detailed calculations, which have been posted online ahead of publication in a scientific journal, came up with a figure of 1,700 cases.
Size of Wuhan outbreak of a novel #coronavirus estimated from the three cases detected outside China: Likely to be over 1000 cases. @imperialcollege @mrc_outbreaks report released today— MRC Centre for Global Infectious Disease Analysis (@MRC_Outbreak) January 17, 2020
🔰 https://t.co/7A77NXZ3iw pic.twitter.com/u0dUnMs9hA
What does it all mean?
Prof Ferguson said it was "too early to be alarmist" but he was "substantially more concerned" than a week ago.
Chinese officials say there have been no cases of the virus spreading from one person to another.
Instead they say the virus has crossed the species barrier and come from infected animals at a seafood and wildlife market in Wuhan.
Prof Ferguson argues: "People should be considering the possibility of substantial human-to-human transmission more seriously than they have so far.
"It would be unlikely in my mind, given what we know about coronaviruses, to have animal exposure, be the principal cause of such a number of human infections."
Understanding how a novel virus is spreading is a crucial part of assessing its threat.
The WHO's China office said the analysis was helpful and would help officials plan the response to the outbreak.
"Much remains to be understood about the new coronavirus," it said. "Not enough is known to draw definitive conclusions about how it is transmitted, the clinical features of the disease, the extent to which it has spread, or its source, which remains unknown."
What is this virus?
Viral samples have been taken from patients and analysed in the laboratory.
And officials in China and the World Health Organization have concluded the infection is a coronavirus.
Coronaviruses are a broad family of viruses, but only six (the new one would make it seven) are known to infect people.
At the mild end they cause the common cold, but severe acute respiratory syndrome (Sars) is a coronavirus that killed 774 of the 8,098 people infected in an outbreak that started in China in 2002.
Analysis of the genetic code of the new virus shows it is more closely related to Sars than any other human coronavirus.
Preliminary analysis of the novel coronavirus (in red) believed to be responsible for an outbreak of pneumonia in Wuhan, China shows it's closely related to SARS CoV. https://t.co/Vf6U2W4oYR pic.twitter.com/3WbFpQ02Pc— EcoHealth Alliance (@EcoHealthNYC) January 11, 2020
The virus has caused pneumonia in some patients and been fatal in two of them.
What do other experts say?
Dr Jeremy Farrar, director of the Wellcome medical research charity, said: "There is more to come from this epidemic.
"Uncertainty and gaps remain, but it's clear that there is some level of person-to-person transmission.
"We are starting to hear of more cases in China and other countries and it is likely, as this modelling shows, that there will be many more cases in a number of countries."
Prof Jonathan Ball, from the University of Nottingham, said: "What's really important is until there has been widespread laboratory testing it is very difficult to put a real number on the cases out there.
"But this is a figure we should take seriously until we know otherwise, 41 animal-to-human 'spillovers' is stretching it a bit and there probably is more underlying infection than has been detected so far."
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