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.
The UK’s chief medical adviser said investigations and contact tracing had begun.
Earlier, Prof Chris Whitty said onward transmission and an epidemic in the UK was “likely”.
“A wave is almost certainly coming, but we don’t know the size,” he said.
The total number of confirmed cases in England is now 80, with three in Scotland and one in Wales and one in Northern Ireland.
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Prof Whitty said the new cases in the UK included 32 patients from England.
“Twenty-nine patients were diagnosed who had recently travelled from recognised countries or from recognised clusters which were under investigation,” he said.
“Three additional patients contracted the virus in the UK and it is not yet clear whether they contracted it directly or indirectly from an individual who had recently returned from abroad.
“This is being investigated and contact tracing has begun.”
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Earlier, Prof Whitty said that up to 80% of the UK population could be infected with coronavirus, which causes the disease Covid-19, “in the worst case scenario”.
But he said a move to lock down entire towns and cities across the UK would be “very unlikely”.
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.
Sick pay
Boris Johnson has said that workers will get statutory sick pay from the first day they are off work to help contain the virus.
In the Commons earlier, he said people who self-isolate should not be “penalised for doing the right thing”.
That 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.