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 41 laboratory-confirmed cases of the new virus, but UK experts estimate the figure is closer to 1,700.
Two people are known to have died from the virus, 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.
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.
Analysis of the genetic code of the new virus shows it is more closely related to SARS than any other human coronavirus.
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."