People across the UK have been told to avoid pubs, clubs, and other social venues as the government ups its "national fightback" against coronavirus.
Some people with underlying health conditions face self-isolation of up to three months from the weekend, as Boris Johnson warned the government was heralding a "very substantial change in the way we want people to live their lives".
The Prime Minister said "now was the time" for Britons to "stop non-essential" contact such as travelling to and working in an office. He said people over 70, pregnant women and those with health conditions should pay particular attention to the advice.
The switch to more drastic measures to contain the spread of coronavirus comes as the number of cases rose to 1,543 with 55 deaths - up from 35 on Sunday - and following criticism of the government's more muted approach over the weekend.
Between 1.2 million and 1.4 million people with underlying health conditions will be shielded from society for up to 12 weeks to avoid them catching the virus.
Speaking at the first of a series of daily press conferences, Mr Johnson also warned that if one person or any member of the household catches a new, persistent cough or a temperature of 37.8 degrees, then all should stay at home for 14 days.
"That means that if possible you should not go out even to buy food or essentials, other than for exercise, and in that case at a safe distance from others," he said.
"If necessary, you should ask for help from others for your daily necessities. And if that is not possible, then you should do what you can to limit your social contact when you leave the house to get supplies."
Mr Johnson also said that from Tuesday, the government would move "emphatically" away from mass gatherings, following the likes of Italy and France which took action earlier.
When pressed, he admitted that the government did have the power to enforce these measures and that the health secretary could even ban hand-shaking if forced to.
But he added: "Most people would accept that we are a mature and grown-up and liberal democracy where people understand very clearly the advice that has been given to them.
"And I think they also understand that what we're saying obviously helps the NHS and helps them, helps us all as individuals and it helps the whole community."
Addressing the inevitable strain the measures will place on ordinary Brits, the Prime Minister said: "I know that many people - including millions of fit and active people over 70 – may feel, listening to what I have just said, that there is something excessive about these measures.
"But I have to say, I believe that they are overwhelmingly worth it to slow the spread of the disease, to reduce the peak, to save life, minimise suffering and to give our NHS the chance to cope."
Rounding up his speech, the Prime Minister said: "Clearly what we're announcing today is a very substantial change in the way we want people to live their lives.
"I cant remember anything like it in my lifetime, I don't think there has been anything like it in peacetime.
"We have to accept that it is a very considerable, behavioural change that we are asking you, we're asking the public, the nation to do.
But I've absolutely no doubt we can do it, we can do it together, everybody understands the need to do it."
