All British households have been told to stay at home except for a strictly limited set of purposes - with police having powers to enforce the instructions with fines.
Shops selling non-essential goods have been ordered to close, along with libraries, playgrounds and outdoor gyms, and places of worship.
Weddings, baptisms and other ceremonies are being stopped, but funerals will continue to go ahead. And all public gatherings of more than two people - except for members of the same household - will be banned.
From Monday evening, people will only be allowed to leave their homes for:
- Shopping for basic necessities “as infrequently as possible”;
- One exercise outing a day - for example a run, walk or cycle;
- Medical needs or caring for a vulnerable person;
- Travelling to and from work, but only if this is absolutely necessary and cannot be done from home.
The new measures came after the UK death toll from Covid-19 reached 335, with a further 46 people who tested positive for the virus dying in England, and foreign secretary Dominic Raab told all Britons overseas to come home now.
They mean the UK following countries like Italy, Spain and France into stringent restrictions on day-to-day life to suppress the virus, after weeks in which Britain has maintained a lighter-touch approach based on voluntary rather than mandatory actions.
Mr Johnson’s decision to step up action, agreed at a meeting of the Cobra emergency committee in Whitehall, came amid widespread concern over large numbers of people congregating at parks, beaches and outdoor markets over the weekend.
In a televised address to the nation recorded in 10 Downing Street, Mr Johnson said: “In this fight we can be in no doubt that each and every one of us is directly enlisted.
“Each and every one of us is now obliged to join together, to halt the spread of this disease, to protect our NHS and to save many many thousands of lives.
“And I know that, as they have in the past so many times, the people of this country will rise to that challenge.
“And we will come through it stronger than ever.
“We will beat the coronavirus and we will beat it together.
“And therefore I urge you at this moment of national emergency to stay at home, protect our NHS and save lives.”
Mr Johnson described coronavirus as “the biggest threat this country has faced for decades”.
He warned: “Without a huge national effort to halt the growth of this virus, there will come a moment when no health service in the world could possibly cope, because there won’t be enough ventilators, enough intensive care beds, enough doctors and nurses.
“And as we have seen elsewhere, in other countries that also have fantastic health care systems, that is the moment of real danger.
“To put it simply - if too many people become seriously unwell at one time, the NHS will be unable to handle it, meaning more people are likely to die, not just from coronavirus but from other illnesses as well.
“So it’s vital to slow the spread of the disease.
“Because that is the way we reduce the number of people needing hospital treatment at any one time, so we can protect the NHS’s ability to cope - and save more lives.”
Speaking alongside a Union flag, Mr Johnson thanked Britons who have complied with advice to stay at home where possible and to maintain “social distancing” when outside.
But he said: “The time has now come for us all to do more.
“From this evening I must give the British people a very simple instruction - you must stay at home.
“Because the critical thing we must do is stop the disease spreading between households.”
And he added: “You should not be meeting friends. If your friends ask you to meet, you should say No.
“You should not be meeting family members who do not live in your home.
“You should not be going shopping except for essentials like food and medicine — and you should do this as little as you can. And use food delivery services where you can.
“If you don’t follow the rules the police will have the powers to enforce them, including through fines and dispersing gatherings.”
Mr Johnson said: “No prime minister wants to enact measures like this.
“I know the damage that this disruption is doing and will do to people’s lives, to their businesses and to their jobs...
"But at present there are just no easy options. The way ahead is hard, and it is still true that many lives will sadly be lost.
"And yet it is also true that there is a clear way through. Day by day we are strengthening our amazing NHS with 7,500 former clinicians now coming back to the service.
"With the time you buy – by simply staying at home - we are increasing our stocks of equipment, we are accelerating our search for treatments, we are pioneering work on a vaccine.
"And we are buying millions of testing kits that will enable us to turn the tide on this invisible killer." (From The Independent Newspaper)