The British government on Monday warned the outbreak of novel coronavirus was a "serious and imminent threat", doubling the number of cases recorded in the country from four to eight.
Anyone with the virus can now be forcibly quarantined and sent into isolation if they are deemed to pose a public health threat, the government said.
Two hospitals have been designated as "isolation" facilities, both of which currently house Britons who have returned from Wuhan, the Chinese city at the centre of the outbreak.
"The incidence or transmission of novel coronavirus constitutes a serious and imminent threat to public health," the health ministry said.
Britain's Press Association news agency quoted a government source saying the new measures had been announced because one person who returned from China on an evacuation flight "was threatening to abscond" from 14-day isolation.
Public Health England said the four new cases included two healthcare workers, adding that it was now urgently trying to identify patients and medics who may have come in contact with them.
The four new patients who tested positive in Britain are all known contacts of a previously confirmed case in France, the UK health ministry said.
Health authorities were unable to say if these are the same cases reported by the French authorities on Saturday when contacted by AFP.
Five UK nationals who tested positive in France came into contact with a Briton who had returned from Singapore and stayed at a ski chalet near Mont Blanc in the French Alps.
The man was originally from Brighton, southeast England, where he was diagnosed on his return before being transferred to London.
A medical centre in Brighton said on Monday it had temporarily closed for "an urgent operational health and safety reason".
An AFP photographer at the scene saw health workers in hazmat suits at the surgery.
"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," Public Health England said.
"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."
The UK health ministry insisted it was "extremely well prepared" to manage and treat cases.
The SARS-like virus, which first emerged at the end of last year, has killed more than 900 people, and spread around the world. Some 40 000 people have been infected in China and 350 elsewhere.
Most of deaths have also been in China.