The latest patient diagnosed with the coronavirus in England is the first to catch it in the UK.
It is unclear whether this was directly or indirectly from someone who recently returned from abroad, England’s chief medical officer said.
The man is a resident of Surrey who had not been abroad recently himself.
It takes the total number of UK cases to 20 and comes after a British man in his 70s became the first UK citizen to die from the virus.
That man, who lived abroad, had been taken to hospital in Japan after catching coronavirus on the Diamond Princess cruise ship, which was quarantined off the port of Yokohama earlier this month.
Confirming the latest UK case, England’s chief medical officer Prof Chris Whitty said the man had been transferred to a specialist NHS infection centre at Guy’s and St Thomas’ Hospital in central London.
He was a patient at Haslemere Health Centre in Surrey which has been closed for “deep cleaning” since Friday morning.
Prof Whitty said the case was being investigated and contact tracing has begun.
The Department of Health and Social Care said the virus was passed on in the UK but the original source was “unclear” and there was no “immediately identifiable link” to overseas travel.
Public Health England said it was working with Surrey County Council to contact people who had “close contact” with the latest coronavirus case.
According to BBC health editor Hugh Pym, Whitehall sources say emergency legislation could be introduced next week if there is a serious escalation in the number of UK cases.
Health minister Helen Whately told BBC Newsnight the UK was “well-prepared” but it is “likely we will see more cases”.
She urged the public to practise “good personal hygiene” and said plans will be published next week setting out the government’s preparations.
Prof Jonathan Ball, from the University of Nottingham, said the Surrey case marks a “new chapter for the UK” and that it is “crucial” to understand the infection’s origin.
“This was always a concern – this is a virus that frequently causes symptoms very similar to mild flu or a common cold, and it’s easily transmitted from person to person. This means it can easily go under the radar,” the virology expert added.