Two healthcare workers have been diagnosed with coronavirus, Public Health England has said, as a GP surgery was closed after it was linked to a UK case of the illness.
County Oak Medical Centre in Brighton - where one of the healthcare workers is reported to be based - has been temporarily shut, with a person in protective clothing seen cleaning the premises on Monday.
Brackley Medical Centre in Northamptonshire says it has also closed "due to a potential coronavirus incident" but insists it does not have a confirmed case.
The four people infected are all understood to have come into contact with a businessman who contracted the virus at a conference in Singapore and was diagnosed in Brighton last week.
Eleven Britons with coronavirus in total are thought to have contracted it from the man, prompting fears he could be a so-called "superspreader".
The UK government has declared that the coronavirus is a "serious and imminent threat to public health" - with new legal powers announced on Monday allowing people with the virus to be forcibly quarantined.
It is understood that the decision came after one of the Britons who returned from Wuhan in China - the source of the coronavirus outbreak - attempted to leave isolation at Arrowe Park Hospital on Merseyside.
In other developments:
The Department of Health said the four people in the UK who have been newly diagnosed with the coronavirus contracted the illness in France.
They are being treated at specialist infection centres at St Thomas' and the Royal Free hospitals in London.
Public Health England's emeritus medical director Paul Cosford told Sky News that the new UK coronavirus cases were "not a surprise" because of their links to an existing case.
He said: "I'm sure we can expect to see some more cases. Our hope is by the intensive action we're taking at the moment, that we can avoid any further spread from these cases."
However, the World Health Organization has warned that the spread of coronavirus to people who have no travel history to China could be "the spark that becomes a bigger fire".
Director general Tedros Adhanom Ghebreyesus said: "In recent days we have seen some concerning instances of onward transmission from people with no travel history to China, like the cases reported in France and the UK.
"The detection of this small number of cases could be the spark that becomes a bigger fire. But for now, it's only a spark.
"Our objective remains containment. We call on all countries to use the window of opportunity we have to prevent a bigger fire."
The WHO has identified 168 labs around the world with the right technology to diagnose the virus - and has sent testing kits to places including Ethiopia, Iran, Uganda and Zambia.
Public Health England medical director Yvonne Doyle said the latest UK cases of coronavirus are "all closely linked to one another".
She added: "Our priority has been to speak to those who have close and sustained contact with confirmed cases, so we can advise them on what they can do to limit the spread of the virus.
"Two of these new cases are healthcare workers and as soon as they were identified, we advised them to self-isolate in order to keep patient contact to a minimum.
"We are now working urgently to identify all patients and other healthcare workers who may have come into close contact, and at this stage we believe this to be a relatively small number."
British Airways says it has cancelled all its flights to Beijing and Shanghai until 31 March in the wake of the coronavirus outbreak, while flights from the two Chinese cities have been cancelled until 1 April.
Meanwhile, St Mary's Independent School in Southampton told Sky News it has been temporarily closed over coronavirus fears.
In a statement, St Mary's said it would remain shut for the next two days "due to a potential risk of coronavirus reported by members of the school community who have become ill, having recently travelled to the region affected".
It added: "The school will undertake an enhanced clean following advice from Public Health England."
Fifteen people linked to the UK have been diagnosed with the coronavirus.
They had been staying in the same chalet in the French Alps as the British businessman who contracted it in Singapore, and are not in a serious condition in hospital in France, authorities said.
A British man also tested positive for coronavirus in Majorca after he had stayed in the same Alps ski resort.
Meanwhile, Alan Steele, who was on honeymooning with his wife on the Diamond Princess cruise liner, was diagnosed while the luxury ship was quarantined off the coast of Japan.
Scientists in China have found the new coronavirus can have an incubation period of up to 24 days after analysing data on more than 1,000 patients from 552 hospitals across the country.
They discovered the median incubation period was three days but ranged from zero to 24 days.
Currently, the World Health Organisation estimates the incubation period to be up to 14 days.