UP TO 10,000 people could already be infected with the killer coronavirus that's sweeping , experts have warned.
The estimate comes amid warnings the new strain is currently "as deadly as Spanish flu" - which killed 50 million people.
Chinese state media confirmed 17 people have now died - up from nine earlier today.
Meanwhile official cases soared with more than 470 confirmed, according to the country's National Health Commission.
But British experts have today warned the true number of cases is more likely to range from 1,000 to 10,000.
Professor Neil Ferguson, an expert in mathematical biology at Imperial College London told reporters in London the death rate for the new strain of coronavirus is "roughly the same as for The Spanish flu epidemic, at around one in 50".
The 1918 outbreak is the most severe pandemic in recent history, wiping out an estimated 50 million people across the world.
Prof Ferguson warned of "more deaths to come" as fellow experts said the outbreak has reached the threshold for an international public health emergency - ahead of today's World Health Organization meeting on the issue.
He warned the new coronavirus strain could already be in the UK - as officials screen and isolate passengers travelling into Heathrow from China.
CLIMBING DEATH TOLL
Deputy Director of the National Health Commission Li Bin said that all deaths had been in the Hubei province, home to Wuhan city where the first illnesses from coronavirus were reported in late December.
Li warned: “The virus is mainly transmitted through the respiratory tract,” Li said. “It may mutate and there is risk of further spread."
Officials have put Wuhan - a city of 11 million, a larger population than London or New York - on lockdown, with people being told to stop travelling and to avoid crowds.
It comes at one of the busiest times of the year in China as people prepare to celebrate the Lunar New Year holidays.
The climbing death toll comes amid reports the '2019-nCoV' strain has spread to other countries and major cities including Beijing, Shanghai and southern Guangdong province.
Taiwan, Hong Kong and the US were the latest countries to confirm a case of the lethal SARS-like virus - after Australian officials said a man tested amid fears he picked up the bug in China was clear.
The new strain has also hit South Korea, Japan, Thailand, Hong Kong, Macau with the World Health Organization predicting it will continue to cross international borders in the coming days.
It comes as Chinese officials confirmed 'novel coronavirus' - which causes pneumonia - can be passed from person to person.
'WORST YET TO COME'
Leading expert, Dr Jeremy Farrar, director of the Wellcome Trust, said that marks a serious shift change in the outbreak.
"This is extremely concerning," he warned. "Person to person transmission has been confirmed and, as expected, we are seeing rapidly increasing case numbers across China, and in more countries, with health care workers infected."
He praised the speed at which the virus has been identified, and the strong global coordination from the WHO, but warned "more is to come from this outbreak".
"With travel being a huge part of the fast approaching Chinese New Year, it is right that concern levels are at the highest level," he added.
"A major concern is the range of symptoms this virus is causing.
"It is clear some people are being affected and are infectious while experiencing only very mild symptoms at or possibly no symptoms at all (asymptomatic).
"This may be masking the true numbers infected and the extent of person to person transmission. It is a matter of urgency to work this out."
IMPENDING GLOBAL CRISIS
The WHO announced it will hold an Emergency Committee meeting in Geneva today, to determine if the outbreak should be classed as a global crisis.
While China has confirmed 440 cases across 13 areas of the country - up from 300 yesterday - another 2,197 people are thought to have come into contact with infected people and have been isolated.
So far 765 of those have been released from observation, Li Bin said, adding there is mounting evidence that the bug is being spread through "respiratory transmission".
Li said: "Recently there has been a big change in the number of cases, which is related to our deepening our understanding of the disease, improving diagnostic methods and optimising the distribution of diagnostic kits."
Fifteen healthcare professionals are among those infected.
However, experts at Imperial College London had previously estimated that around 1,700 people had been infected.
But today they more than doubled that figure to suggest that the true figure was closer to 4,000.
Prof Ferguson told a briefing in London today: "We are doubling our estimate from 1,700.
"We estimate around 4,000 will have been sick in Wuhan and developed symptoms by the 18th January.
"The reason that's more than double - the calculation hasn't changed - it's now because by that cut off time we have seven cases reported outside mainland China."
WHO's regional director for the western Pacific, Takeshi Kasai, said: "Information about newly reported infections suggest there may now be sustained human-to-human transmission."
The mayor of Wuhan confirmed the latest deaths today, an 89-year-old man from the city, which is home to more than 11 million people.
A 66-year-old man, known only as Li, and a woman, 48, known as Yin were also confirmed to have died from multiple organ failure.
In Taiwan today, an epidemic response command centre has been set up with more than 1,000 beds prepared in isolation wards in case the virus spreads further.
There, health officials confirmed a woman, thought to be in her 50s, had caught the new strain.
She is currently in hospital and receiving treatment, according to local media reports.
So far, the WHO has not advised travel or trade restrictions but could put such measures in place at tomorrow's emergency meeting.
Millions pose travel threat
It comes as China faces its busiest travel period over Chinese New Year - a time when millions board trains and planes to celebrate the Lunar New Year holidays.
Speaking on LBC today, leading virologist Professor John Oxford, from Queen Mary College, said he was "quaking in my shoes" at the potential for spread over the holidays.
He said: "None of us have faced a new virus with so many people in a community travelling around.
"That's what's going to happen in China at the end of the week.
"Once they are close together in taxis or small rooms, then there may be a problem.
"The only way to stop it is physical cleaning and social distance - keeping away from people."
WHO chiefs are set to consider if this outbreak should be declared a global emergency like the 2014 Ebola outbreak in West Africa, the current outbreak in Congo and the Zika outbreak in Central America in 2016.
While NHS chiefs have urged doctors in the UK to be alert to signs of the killer virus, they said today the threat to Brits is "low".
The origin of the virus is not get known, but experts say the most likely source is an animal.
Chinese officials have linked the outbreak last month to a seafood market in the city of Wuhan - the epicentre of the current outbreak, where more than 270 cases have been confirmed.
The new strain belongs to a large family of viruses that cause illnesses ranging from the common cold to more severe diseases such as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS).
The common early signs of infection include fever, cough, shortness of breath and breathing difficulties, according to WHO.
In more severe cases, it can cause pneumonia, severe acute respiratory syndrome, kidney failure and even death.
GLOBAL AIRPORT SCREENING
In Australia, border forces have been ordered to ensure all sick passengers are assessed by a trained Biosecurity officer on arrival.
Chief health officer Brendan Murphy said the risk was low but added the three daily flights from Wuhan will be met by medics in response to the "rapidly emerging situation".
State health officials in New South Wales are distributing pamphlets in English and Chinese to all passengers arriving from Wuhan describing the symptoms.
The crackdown comes as it was revealed the virus was feared to have reached Australia after a man was tested for the killer mystery bug.
The man, from Brisbane, was displaying symptoms of the fatal coronavirus after returning from a family holiday and has been placed in quarantine.
The US has also started screening passengers on flights from Wuhan arriving at John F. Kennedy International Airport, San Francisco International Airport and Los Angeles International airport.
Other international airports are also screening passengers for the mysterious SARS-like disease after it was revealed it had jumped China’s borders.
A video has been shared widely on Twitter showing people on a domestic flight out of the central Chinese city of Wuhan having their temperature taken one-by-one by people in protective suits.
A Brit on holiday in Thailand is feared to be the first western victim of the illness.
Ash Shorley, 32, was rushed to hospital after the bug infected both lungs while he was on Koh Phi Phi island in Thailand.
The outbreak has caused alarm because of its connection to Severe Acute Respiratory Syndrome (SARS), which killed nearly 650 people across mainland China and Hong Kong in 2002-2003.
What is coronavirus?
Coronavirus is an airborne virus, spread in a similar way to colds and the flu.
The virus attacks the respiratory system, causing lung lesions.
Symptoms include a runny nose, headache, cough and fever, shortness of breath, chills and body aches.
It is incredibly contagious and is spread through contact with anything the virus is on as well as infected breath, coughs or sneezes.
Symptoms include a runny nose, headache, cough and fever, shortness of breath, chills and body aches.
In most cases, you won't know whether you have a coronavirus or a different cold-causing virus, such as rhinovirus.
But if a coronavirus infection spreads to the lower respiratory tract (your windpipe and your lungs), it can cause pneumonia, especially in older people, people with heart disease or people with weakened immune systems.
There is no vaccine for coronavirus.
In 2003 an outbreak of a similar virus, SARS, infected more than 8,000 people in 37 countries before it was brought under control, killing 800 of those worldwide.
Coronaviruses cause diseases ranging from the common cold to more severe ones such as SARS.
The Chinese government initially tried to conceal the severity of that epidemic but its cover-up was exposed by a high-ranking physician.
Initial symptoms of the novel coronavirus include fever, cough, tightness of the chest and shortness of breath, and those seriously ill developed pneumonia.
At least half a dozen Asian countries and three US airports have started screening incoming airline passengers from central China.
MOST READ IN NEWS
Chinese Xi Jinping said it was “extremely crucial” to take every possible measure to combat the disease that has infected at least 217 people in the country.
He said: "The recent outbreak of novel coronavirus pneumonia in Wuhan and other places must be taken seriously
“Party committees, governments and relevant departments at all levels should put people’s lives and health first.”