Boris Johnson plunged Britain into lockdown today - urging everyone in the country to stop all "non-essential" contact with others.
The PM warned that the coronavirus was now in a phase of rapid spread across the UK, and it was time to take radical action to stop the NHS being swamped.
Anyone in a household where someone has been showing symptoms should isolate, he said. And the rest of Britain should avoid all contact that was not necessary - with restaurants, bars and cinemas and travel off limits, and a ban on large gatherings.
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At a press conference in Downing St, Johnson said: "Now is the time for everyone to stop non-essential contact with others."
Admitting that the squeeze could last 12 weeks, Johnson said that meant "you should not go out, even to buy food or essentials" - and he acknowledged he was "asking a lot".
"If necessary, you should ask for help from others for your daily necessities. If that is not possible, you should do what you can to limit your social contact when you leave the house to get supplies," he said.
"Now is the time for everyone to stop non-essential contact with others and stop all non-essential travel. We need people to start working from home where they possible can. You should avoid pubs, clubs, theatres and other such social venues."
In a special plea to the capital, Johnson said people there were at the highest risk. "It looks as though London is now a few weeks ahead ... it's important that Londoners now pay special attention to what we are saying about avoiding all non-essential contact."
The dramatic move came amid growing criticism of the UK government's response, which has looked increasingly out of step with that around the globe. However, Johnson said he was not yet ordering schools to close.
"We think at the moment on balance it is much better if we can keep schools open for all sorts of reasons," he said.
"But this is something that we need to keep under review."
There were also immediate questions over how the government would be able to enforce the lockdown.
Johnson said the UK government had sweeping powers if they needed them, and the Health Secretary could even "ban handshakes" if needed.
"But most people would accept that that we are a mature liberal democracy where people understand very clearly the advice that is being given to them," he said.
The World Health Organisation earlier swiped at the abandonment of testing for those with symptoms, saying it was like trying to "fight a fire blindfolded".
Britain's chief medical officer Chris Whitty, speaking alongside Johnson and chief scientific officer Patrick Vallance in Downing Street, said the restrictions on families were crucial.
"If one person in the household has symptoms the whole household stays at home," Whitty said.
Fears over the impact of the coronavirus were laid bare in a leaked Public Health England (PHE) briefing warning that a "worst case" scenario could see an epidemic last until spring next year, and mean 7.9 million needing hospital treatment.
Downing St stressed the 7.9 million figure was just the reasonable worst case scenario and "does not mean that is what we expect to happen".
France has imposed controls on its border with Germany, and transport restrictions are in force across much of Europe, with schools, pubs and restaurants closed. Austria is banning gatherings of more than five people. The US has barred all travellers from Europe.
However, beyond urging those with a cough or a fever to self-isolate, the UK authorities have up to now held off the most draconian steps, saying the timing must be right for them to be effective.
Earlier, Transport Secretary Grant Shapps conceded that Britain will soon need the same tough steps, as it was only a "little behind" neighbours such as France and Germany in the progress of the disease.
The British public appears to be taking matters into their own hands today, with commuter trains unusually empty as workers opt to stay at home. Shapps suggested "ghost" services could be halted altogether, as airlines announced more cancellations and demanded a bailout.
Amid rising criticism, Johnson is to chair another Cobra emergency meeting later, and daily press briefings will be staged to reassure the public everything possible is being done.
Ministers have appealed for major manufacturers to overhaul their factories to produce ventilators to help those who are most at risk from the illness.
Under tough new powers, people who refuse to go into quarantine face being detained or slapped with a fine of up to £1000 ($2000).
Police are able to use "reasonable force" to constrain those who could infect others.
Pushed on the relative lack of UK action so far, Shapps told Sky News: "The UK has probably just been at a slightly different stage - compared with places like Italy but also a little behind where France and Germany are.
"It's not that we're not going to get there, but of course our responses are timed in a different way, unique to the particular stage of this that we're in in the UK.'
Shapps told BBC Radio 4's Today programme services would be maintained, but added there is no point running "ghost trains".
He also played down criticism over the testing regime in the UK, saying the government is carrying out more tests than any other state apart from Italy and China.
"We are working very hard to do things including, for example, providing the ability for home testing, but just in terms of scientific technology - getting those tests available, in other words inventing tests that can be used that quickly and in that way - they are not available at this time," he said.
"Having said that, the UK - I think I'm right in saying other than China and Italy - is carrying out more tests than any other country in the world, so we are ahead of the game in terms of testing."
The Transport Secretary said people over the age of 70 will be asked to self-isolate "as and when the moment is right" - but that they would still be able to go outside and "walk the dog".
He said it is "quite likely" that elderly people would have to self-isolate for months, but added: "It is the case that people will be able to go out and walk the dog. It's about being sensible but not mixing in crowds."
However, the UK appears increasingly out of step with other countries in terms of its response to the emergency.
Singapore foreign minister Lawrence Wong suggested Britain's abandonment of efforts to "contain" coronavirus will lead to cases rising "even more sharply".
He told a press conference: "In the case of the UK it is rising very rapidly but it's not just about the numbers but the fact that these countries have abandoned any attempt at containing the spread of the virus.
"They have said so publicly, especially the UK and Switzerland, perhaps less so for Japan; but certainly in the case of the UK and Switzerland.
"I think the UK has been most public in acknowledging that there's no point containing and they are simply now at the phase of trying to delay the spread.
"So if there is no deliberate effort to contain then we anticipate that the numbers of infected cases in these countries will rise even more sharply in the coming days or weeks, and that's why we are quite concerned."