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 45 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.