Britain on Thursday said up to 10,000 people in the UK could be infected with the novel coronavirus, as it announced new measures to slow the spread of the outbreak.
Government chief scientific adviser Patrick Vallance said there were currently 590 confirmed cases but it was "much more likely we've got somewhere between 5,000 and 10,000 people infected".
Britain was "about four weeks" behind the level of cases currently seen in Italy and other worst-hit countries in Europe, he added.
Vallance was speaking after Prime Minister Boris Johnson called the outbreak, which has so far killed 10 people in Britain, "the worst public health crisis for a generation" and warned it would get worse.
"Many more families are going to lose loved ones before their time," he told a news conference at his Downing Street office.
He announced new measures to combat the rapid spread of the virus, recommending anyone with COVID-19 symptoms stays at home for at least seven days.
He also said schools will be asked to cancel foreign trips and elderly people or those in poor health were told not to go on cruise ships.
The move is designed to delay the peak of the virus to ease pressure on the state-run National Health Service, which is already stretched by winter flu outbreaks.
Stretching the peak over a longer period would mean "more beds and more time for medical research... so society is better able to cope", Johnson added.
The British premier said further measures were likely in the coming weeks, including the possibility of cancelling mass gatherings such as sporting events.
UK health officials said most of the 5,000 to 10,000 people will not be aware that they have COVID-19 but it was clear the virus was circulating in the community.
Britain's response has been criticised for not going as far as other countries, including Italy, which has seen towns and cities quarantined, schools and shops shut and travel banned.
Johnson has repeatedly insisted that hand-washing is the best defence against contracting the virus and maintained the government is being guided by his scientific advisers.
He reiterated Thursday that scientists advised against school closures, even as neighbouring Ireland shut all its schools, colleges, childcare facilities, and banned mass gatherings.
"Schools should only shut if they are specifically advised to do so," he told reporters.
Separately, Scotland's First Minister Nicola Sturgeon said the devolved administration in Edinburgh would recommend banning mass gatherings, including football and rugby, from Monday.
But Johnson and his advisers stopped short of following suit, saying the risk from crowds was lower than in smaller venues.
On Wednesday, a former senior official at Public Health England accused the government of complacency.
"If this now spreads the way it looks likely to spread, there will not be enough hospital beds and people will have to be nursed at home," John Ashton told BBC television.
"We should have gotten a grip on this a month ago." Ashton's criticism follows that of the editor of The Lancet medical journal, Richard Horton, who accused Johnson and health minister Matt Hancock of "playing roulette with the public".
He said more aggressive "social distancing and closure policies" should have been introduced sooner.
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