There is an “increased likelihood” of cases of the new coronavirus occurring in the UK, Health Secretary Matt Hancock has said.
Globally, there are more than 500 confirmed cases of the virus, which has killed 18 people in China.
But there are no known cases in the UK, Mr Hancock said, which was “well prepared” to deal with an outbreak.
However, four people are being tested in hospitals in Scotland and Northern Ireland after showing symptoms.
All the patients had been in Wuhan – the Chinese city where the new strain of the virus, which can cause lung disease such as pneumonia, first emerged – in the last 14 days.
One man is being treated in the Royal Victoria Hospital in Belfast – he is thought to have been admitted with a high temperature.
The Scottish government confirmed there were three suspected cases in Scotland. There had previously been thought to be a fourth case.
Prime Minister Boris Johnson’s spokesman said the tests were “purely precautionary”.
It is likely to be several days before tests will confirm whether they have contracted coronavirus or not.
Wuhan – which has a population of 11 million people – has gone into lockdown, with authorities suspending planes and trains in and out of the city.
The UK is monitoring flights arriving from China as a precaution.
In a statement to the House of Commons, Mr Hancock said that it was a “rapidly developing situation and the number of deaths and the number of cases is likely to be higher than those that have been confirmed so far and I expect them to rise further”.
He told MPs: “The chief medical officer has revised the risk to the UK population from ‘very low’ to ‘low’ and has concluded that while there is an increased likelihood that cases may arise in this country, we are well prepared and well equipped to deal with them.”
He added: “The UK is one of the first countries to have developed a world-leading test for the new coronavirus.
“The NHS is ready to respond appropriately to any cases that emerge.”
In response to the outbreak, the Foreign Office has advised against all but essential travel to Wuhan.
On Wednesday, Public Health England began carrying out enhanced monitoring of direct flights from China.
Passengers are receiving advice on what to do if they fall ill, which Mr Hancock said was the most important part of the monitoring as it can take days after infection before a patient develops symptoms, so physical checks were considered less useful.
He added: “We are working closely with our counterparts in the devolved administrations and the public can be assured that the whole of the UK is always well prepared for these types of outbreaks.
“And we’ll remain vigilant and keep our response under constant review in the light of emerging scientific evidence.”
Authorities around the world have announced screening measures for passengers from China.
Singapore, Saudi Arabia and Vietnam are the latest countries to have registered a confirmed case.
Thailand has confirmed four cases of the virus – the most outside of China. The US, Japan, Taiwan and South Korea have all reported one case each.
The World Health Organization’s (WHO) emergency committee is meeting on Thursday in Geneva to decide whether or not to declare a “global emergency”.
A global emergency is the highest level of alarm the WHO can sound and has previously been used in response to swine flu, Zika virus and Ebola.
All but one of the fatalities so far have been in Hubei province, of which Wuhan is the capital. Most of the 17 victims were elderly and suffered from other chronic diseases including Parkinson’s disease and diabetes.
.The lockdown comes as millions of Chinese people travel across the country for the upcoming Lunar New Year holiday.