Officials are trying to work out how a man in Surrey became the first coronavirus patient to catch the disease within the UK.
Unlike the other 19 Covid-19 positive cases in the country, the unnamed man has not travelled aboard recently.
All previous confirmed cases have been passed on overseas, in countries including France, Italy and Iran.
The hunt is now on to find people who have been in contact with the man in a bid to stop the virus spreading further.
Yesterday the first British person died from the disease while on the Diamond Princess cruise ship which is quarantined in Japan.
Health experts have said that tracing the origins of the latest UK case of coronavirus will be "crucial" in preventing more extensive spread.
Jonathan Ball, professor of molecular virology at the University of Nottingham, said UK transmission was "always a real possibility" and that with symptoms very similar to flu, it can be easy for Covid-19 to go under the radar.
He said: "This case – a person testing positive for novel coronavirus with no known link to an affected area or known case – marks a new chapter for the UK and it will be crucial to understand where the infection came from to try to prevent more extensive spread.
"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."
Dr Stephen Griffin, associate professor at the University of Leeds, said it had been "only a matter of time" before the first person-to-person transmission in the UK was confirmed.
"What now becomes critically important is our ability to identify, isolate and care for infected individuals, and to trace their recent contacts," he said.
"If localised outbreaks remain contained then it should be possible to limit the impact upon the UK.
"However, I suspect the frequency of small outbreaks might increase in coming weeks as the source countries of imported cases become more diverse.
"If we experience a burgeoning epidemic as seen in South Korea it will represent a significant challenge to our already stretched NHS and public health infrastructure.
"As directed by WHO, we should use this time window to invest and prepare for such a potential outbreak, whether it happens or not."
Health minister Edward Argar expressed confidence that the disease could be contained.
He told BBC Radio 4's Today Programme: "We're still in the containment phase of this disease. We have been pretty good at containing it thus far.
"And the chief medical officer has been very clear that there is no reason to think that we shouldn't be able to continue containing it, so that's what our focus is on."
As the disease's spread across the EU gains pace, the UK government looks to bring in emergency laws to keep the country moving.
Crisis legislation to grant sweeping new powers to public bodies are being considered by senior MPs, according to reports.
The government's emergency COBRA committee will finally meet on Monday after Boris Johnson agreed to chair crisis talks.
Measures being considered reportedly include letting schools scrap maximum class size and holding lessons outside school in the event of widespread staff absences.
Lorry drivers' 56-hour-a-week time behind the wheel could also be scrapped and army doctors could be sent into hospitals as a "worst case scenario" if NHS staff levels collapse.
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