All flights out of a city in China hit with coronavirus have been cancelled as the World Health Organisation delayed its decision on whether to declare an international public health emergency.
It comes as the UK Foreign Office issued a warning against "all but essential" travel to the area after the Government announced all direct flights from Wuhan into Heathrow would be subjected to enhanced monitoring.
Chinese state media said outbound flights and trains have been halted in Wuhan, the capital of Hubei province, and citizens have been advised not to leave the city as the coronavirus continues to spread.
At least 540 people have contracted the mysterious Sars-like virus, which so far has killed 17 people, Reuters reported.
Other cases have been confirmed in Japan, Taiwan, South Korea, Thailand and the US.
WHO said it will decide on Thursday whether to declare a global emergency over the outbreak of a new flu-like virus spreading in and beyond China.
Director-general Tedros Adhanom Ghebreyesus said they needed more time before making a decision, which was expected on Wednesday evening.
He said: "I have decided to ask the emergency committee to meet again tomorrow to continue the discussion and the chair has agreed with that request."
"The decision about whether or not to declare a public health emergency of international concern is one I take extremely seriously, and one I am only prepared to make with appropriate consideration of all the evidence," he added.
He was speaking after the WHO held a day-long meeting of an independent panel of experts in Geneva on Wednesday.
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The WHO's head of emergencies programme, Mike Ryan, said the priority now was to find the roots of how the virus is passing between people.
Earlier on Wednesday, the Department of Health and Social Care (DHSC) said precautionary measures were being put in place at Heathrow after cases of the virus spread to other parts of the world.
There are three direct flights a week from Wuhan in China to Heathrow Airport, landing at around 6pm on Monday, Wednesday and Friday.
Under the new measures, it was planned that planes would be taken to an isolated area of terminal four.
The captain of each flight would then tell passengers during landing to let a flight attendant know if they feel unwell, and these details would then be passed on to public health teams at the airport who would carry out further checks.
Meanwhile, the Foreign and Commonwealth Office updated its travel advice for China, with a spokesman saying: "In light of the latest medical information, including reports of some person-to-person transmission, and the Chinese authorities' own advice, we are now advising against all but essential travel to Wuhan.
“The safety and security of British nationals is always our primary concern, and we advise British nationals travelling to China to remain vigilant and check our travel advice on gov.uk.”
Public Health England (PHE) upgraded the risk to the UK population from coronavirus from "very low" to "low".
Dr Nick Phin, deputy director of the National Infection Service at PHE, said: "This is a new and rapidly evolving situation where information on cases and the virus is being gathered and assessed daily.
"Based on the available evidence, the current risk to the UK is considered low.
"If you are travelling to Wuhan, you should maintain good hand, respiratory and personal hygiene, and should avoid visiting animal and bird markets or people who are ill with respiratory symptoms.
"Individuals should seek medical attention if they develop respiratory symptoms within 14 days of visiting Wuhan, either in China or on their return to the UK.
"They should phone ahead before attending any health services and mention their recent travel to the city."
The Associated Press reported the state-owned People's Daily newspaper said in a tweet no-one would be allowed to leave Wuhan starting at 10am and that train stations and the airport will shut down.
The newspaper is also reported to have said that Wuhan authorities are saying city buses, subways, ferries and long-distance shuttle buses would also be temporarily closed.
Professor Neil Ferguson, director of the Medical Research Centre for Global Infectious Disease Analysis, said the estimated number of people infected with coronavirus in Wuhan is around 4,000, with a range between 1,000 and 9,700.
Asked whether it was possible the virus had already reached the UK, Prof Ferguson said he could not rule it out.
He said: "We can't rule out that possibility.
"Border screening, and in this case, in the UK, alerting the health system is not 100% foolproof - there could be a mild case.
"The UK is not a major destination of visitors travelling out of Wuhan around the world though.
"So I would say it was unlikely, but can't be completely sure."
Prof Ferguson said "trying to identify people who are sick coming off a plane will only identify, if you're lucky, people who will have fever coming off a plane.
"If somebody was infected two days before they travelled, they will arrive without any symptoms at all.
"That's why it's essential that actually the entire... health system is alert to the possibility of imported cases."