Britons with the most serious health conditions should be "largely shielded from social contact" for three months to avoid coronavirus.
That was the advice given by UK prime minister Boris Johnson as he called on Britons to "stop all non-essential contact and unnecessary travel".
He said the spread of the virus was particularly prevalent in the capital and that Londoners should work from home and avoid pubs and restaurants.
But he stopped short of imposing the kind of restrictions seen in France, where pubs, schools and restaurants have been closed, or Italy, under a complete lockdown.
"This is a very fast-moving situation and the latest numbers suggest we are entering a fast-growth period or on the cusp of doing so," said the UK's chief scientific advisor, Patrick Vallance. "London is ahead of parts of the country."
Johnson said the number of UK cases was starting to rise rapidly and ``without drastic action`` "they could double every five or six days". As of Monday, Britain had 1,543 confirmed cases and 53 virus-related deaths.
British authorities now say if anyone in a household has a fever or persistent cough, everyone there should stay at home for 14 days.
Johnson also said the government would no longer give emergency-services support to large gatherings, though he did not ban them outright. And unlike schools in most other European countries, those in Britain remain open.
``"We think that, on balance, best to keep schools open but appreciate that this is something we need to keep under review,'' Johnson said.
Britain lags behind countries such as Italy, Germany and France in the number of infections, and the government's scientific advisers have said that implementing draconian measures too early will make them harder to sustain as the outbreak peaks in two or three months.
Until Monday, Britons had been told merely to wash their hands frequently and to stay at home for a week if they have a fever or continuous cough.
The U.K. strategy is based on the presumption that most people will eventually get the virus. Britain's goal is to slow the spread of the infection so the country's overstretched National Health Service is not overwhelmed while protecting those most at risk of serious illness: the elderly and people with serious health problems.
Britain's approach had come under increasing pressure as neighboring countries went into lockdowns and closed their borders in response to the new virus. Some scientists urged more severe restrictions to enforce ``"social distancing" and slow the spread of the virus.
For most people, the new coronavirus causes only mild or moderate symptoms, such as fever and cough, and most recover. The worldwide outbreak has sickened over 179,000 people and left more than 7,000 dead. Over 78,000 people have recovered, most of them in China.
The outbreak has already had a huge effect on everyday life in Britain. Ridership on trains and the London Underground is down by a fifth as some businesses ask staff to work from home. Universities are moving classes online and several of London's West End theatres have shut down, with more expected to follow.
Supermarkets have been stripped of staples including toilet paper, pasta and rice as shoppers ignore government appeals not to hoard supplies.
Airlines including easyJet, Ryanair, Virgin Atlantic and British Airways say they will ground most of their planes as more and more countries impose travel restrictions and shut their borders.
Johnson said the measures announced on Monday would affect the lives of everyone in the country and were unprecedented in peacetime.
`"And we have to accept that it is a very considerable psychological, behavioural change that we're asking you, we're asking the public, the nation to do,'' he said. ``But I have absolutely no doubt that we can do it, that we can do it together.''