A total of 87 people in the UK have now been diagnosed with coronavirus, the Department of Health said – a jump of 34 in just a few hours.
The figures represent the biggest day-on-day increase, with 32 positive tests emerging in England and two in Northern Ireland.
Three of the 32 new cases recorded in England were passed on in the UK, raising fears that community transmission may now be taking hold.
England’s chief medical officer, Professor Chris Whitty, said: “As of 9am this morning 32 further patients in England have tested positive for Covid-19.
“Twenty-nine patients were diagnosed who had recently travelled from recognised countries or from recognised clusters which were under investigation.
“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.
“The total number of confirmed cases in England is now 80. Following previously reported confirmed cases in Northern Ireland, Wales and Scotland, the total number of UK cases is 85.”
Earlier, Prof Whitty told the BBC there could be a need to do “extreme things” to protect the elderly and those with pre-existing health conditions.
He added: “At this point in time we think it is likely, not definite, that we will move into onward transmission and an epidemic here in the UK.”
But he stressed that for most people, “this will be a mild or moderate disease, anything from a sniffle to having to go to bed for a few days, rather like with mild flu”.
On the NHS, Prof Whitty said: “The NHS will always cope because the NHS is an emergency service which is very good at adapting to what it finds itself with.”
But he said if the UK sees a very large epidemic, “then it will put very high pressure on the NHS”, and there could be “several weeks which could be very difficult” for the health service and wider society.
Prof Whitty suggested that shutting down cities in the UK would not be effective now.
He said: “Closing cities is really only appropriate if you have a significant epidemic in one particular place and almost nothing anywhere else.
“It made sense for China to respond in the way it did but it would be very unlikely here … This is now in multiple places in Europe and around the world.”
As cases in the UK climb, the Department of Health said it would no “longer be tweeting information on the location of each new case”.
It said: “Instead, this information will be released centrally in a consolidated format online, once a week. We are working on this now and plan to share on Friday.”
But it is known that of the new cases reported, two people are from Carlisle and both had recently returned from a trip to northern Italy.
One is a member of healthcare staff at the Cumberland Infirmary.
Another patient who was admitted to Wythenshawe Hospital in Manchester has also tested positive for Covid-19.
Northern Ireland later confirmed two more cases, taking the UK total to 87.
Liverpool City Council said the first case of coronavirus had been confirmed in the city.
The resident became infected while in Italy and was receiving specialist treatment, according to the council.
Elsewhere, the release date for No Time To Die has been pushed back from April to November following “careful consideration and thorough evaluation of the global theatrical marketplace”, James Bond producers have announced.
The London Book Fair at Olympia from March 10 to 12 was cancelled after major publishers pulled out amid coronavirus fears.