The number of coronavirus cases in the UK has risen by 34 in a day, to a total of 85.
Twenty-nine had recently travelled to affected countries or picked it up from people known to have travelled from abroad.
But three more contracted the virus in the UK and no one knows where they got it from.
England’s chief medical officer said investigations and contact tracing have begun.
The total number of confirmed cases in England is now 80, with three in Scotland and one in Wales and one in Northern Ireland.
Health Secretary Matt Hancock will chair a Cobra meeting at 17:00 GMT, Downing Street said.
What do I need to know about the coronavirus?
- HOW IS THE UK PLANNING FOR AN OUTBREAK: How prepared are we?
- WHAT ARE THE SYMPTOMS? A simple guide
- HOW TO SELF-ISOLATE: What you need to know
- WHERE ARE WE WITH A VACCINE? Progress so far
- A VISUAL GUIDE TO THE OUTBREAK: Virus maps and charts
England’s chief medical officer Prof Chris Whitty said earlier that onward transmission and an epidemic in the UK was “likely”.
In “the worst case scenario”, up to 80% of the UK population could be infected with coronavirus, which causes the disease Covid-19, he said.
But he said a move to lock down entire towns and cities across the UK would be “very unlikely”.
In other key developments:
- Italy is considering closing schools and universities across the country until mid-March – a decision will be taken “in the next few hours”, its education minister says. Italy is the worst-hit European country with some 2,260 cases and 79 deaths so far – including more than 20 in the past 24 hours
- Guests who left a quarantined hospital in Tenerife have been told to self-isolate for a further week after a British woman at the hotel tested positive on 2 March
- CCTV images have been released of four men wanted over an alleged racist attack on London’s Oxford Street, reportedly motivated by virus fears
- There are fears of global shortages of some common drugs after India limited the export of certain medicines
- Grants and low-cost loans are part of a $12bn (£9.4bn) emergency aid package to help developing countries
- And Australians have been stockpiling toilet paper after the country’s first death was reported
Hospitals in England are being asked to carry out more consultations by video to reduce the risk of the virus spreading.
A major public health campaign urging people to wash their hands regularly for at least 20 seconds has also been launched.
Boris Johnson has said workers will get statutory sick pay from the first day they are off work, not the fourth, to help contain the virus.
In the Commons, the prime minister said people who self-isolate should not be “penalised for doing the right thing”.
The sick pay announcement means those receiving statutory sick pay would get an extra £40 a week.
Officials believe up to a fifth of the workforce may be off sick during the peak of an epidemic in the UK.