Prime Minister Boris Johnson warned on Thursday that many more families would see their loved ones die from coronavirus, as the government's chief scientific adviser said Britain likely has as many as 10,000 people infected.
Britain and the EU also agreed to cancel face-to-face trade negotiations planned for next week in London due to the outbreak.
At a meeting of the government's emergency committee on Thursday, Johnson moved Britain's response to the outbreak to the so-called "delay phase", aimed at putting off the peak until the summer months, ending a phase of outright containment.
"It's going to spread further," Johnson said at a news conference, flanked by the government's top scientific and medical advisers.
"I must level with you, level with the British public - more families, many more families are going to lose loved ones before their time."
Rapidly spreading disruptions to business, trade and everyday life around the world have triggered panic on global markets, including in Britain.
Britain's FTSE 100 was down 10.9%, its worst one-day performance since 1987. Sterling fell three cents against the dollar, the biggest fall since the 2016 vote to leave the EU.
The British government has faced questions over why it is not taking the more far-reaching measures seen in many other countries. Johnson defended his approach, saying the government was following scientific advice and would "do the right thing at the right time".
He said those showing even mild symptoms of having the virus should self-isolate for at least seven days. In the next few weeks that advice would change to entire households being asked to stay at home if one person has symptoms.
"This is the worst public health crisis for a generation," he said.
FOUR WEEKS BEHIND ITALY
The number of confirmed cases of coronavirus across the United Kingdom rose 29% percent to 590 over the past 24 hours. Ten people with coronavirus have died in the UK.
Graphic: UK coronavirus cases - https://fingfx.thomsonreuters.com/gfx/polling/1/698/688/coronavirus%20thurs.png
"There are currently about 590 cases identified in the UK and there are more than 20 patients on intensive care units," chief scientific adviser Patrick Vallance said.
"If you calculate what that really means in terms of the total number, it is much more likely that you have somewhere between 5,000 and 10,000 people infected at the moment."
Vallance said Britain was currently on a trajectory about four weeks behind Italy, which has reported more than 15,000 cases and 1,000 deaths. He predicted the peak of coronavirus cases in Britain may be at least 10 to 14 weeks away.
England's chief medical adviser Chris Whitty said not all suspected cases of the virus would be formally tested, with the government focusing on those in hospital.
The British plan for dealing with the outbreak has three main phases - containment, delay and mitigation.
The delay phase includes consideration of so-called social distancing measures such as more home-working, reducing large scale gatherings and closing schools.
Johnson said Britain was not cancelling major public events and schools would stay open for now but that advice could change as the virus spreads.
Whitty said at some point social interaction for old and vulnerable people would need to be reduced, but not yet.
"People start off with the best of intentions but enthusiasm at a certain point starts to flag," Whitty said. "So we do need to do it at the last point it is reasonable ... to get through what will be quite difficult things to do."
Meanwhile, Ireland is to shut schools, universities and childcare facilities until March 29 and restrict mass gatherings, acting Prime Minister Leo Varadkar said.
(Additional reporting by William James, Elizabeth Howcroft, Kate Holton and Andy Bruce, editing by Guy Faulconbridge and Stephen Addison)
