The prime minister urged people to work from home and avoid pubs, clubs and theatres as he held the first in a series of daily Downing Street press conferences.
“This advice about avoiding all social contact is particularly important for people over 70, for pregnant women and for those with some health conditions,” Johnson said.
He also told all those with a persistent cough or high temperature to self-isolate for 14 days, and for their families to join them in doing it.
The government will also no longer support mass gatherings with emergency workers.
Mass gatherings are something “we are now moving emphatically away from,” Johnson said.
Johnson said, by the weekend, groups particularly vulnerable to Covid-19 - over-70s with underlying conditions - will be asked to stay at home for 12 weeks to “ensure this period of shielding, this period of maximum protection coincides with the peak of the disease”.
The PM said the “draconian” measures were required as the outbreak was growing quickly on an “upward curve” and “without drastic action cases could double every five or six days”.
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London is “a few weeks ahead” of the rest of the country and people in the capital should pay special attention to the measures, including home working, suggested by the government, Johnson added.
But despite the new measures, the government has not yet closed schools though this could change in the coming days.
Chief medical officer for England professor Chris Whitty said the measures would need to be in place for a “prolonged period”.
He said experts had examined the proposals – looking at mathematical models and behavioural sciences – to find the measures which were the most effective but were also sustainable.
“This is going to go on for some time,” he said. “We should not be under any illusions that if we just do this for a couple of weeks that is sufficient.
“This is going to have to be a prolonged period.”
Chief scientific adviser Sir Patrick Vallance said the moves would have a “big effect” to bring down death rates and the peak of the disease.
“This is not a series of small interventions. You would anticipate that this could have a dramatic effect to reduce the peak and to reduce death rates,” he said.
“They are not easy but they are important and they will have the effect if we all do it.
“This is a matter for us to take accountability to make sure we help each other, protect ourselves and protect the NHS.”
The prime minister concluded: “I know that many people including many millions of fit and active people over 70 may feel listening to what I have just said is that there is something excessive about these measures.
“But I have to say I believe 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.”
He went on: “I know that we are today asking a lot of everybody – this is far more now than just washing your hands, though clearly washing your hands remains important.
“But I can tell you, across this country people and businesses in my experience are responding with amazing energy and creativity to the challenge we face and I want to thank everybody for the part that you are playing and that you are going to play.”
Speaking in the Commons after the press conference, health secretary Matt Hancock told MPs the measures announced were “unprecedented in peacetime”.
He said: “We will fight this virus with everything we’ve got.
“We are in a war against an invisible killer and we’ve got to do everything we can to stop it.”
After the World Health Organisation (WHO) called on all countries to “test, test, test” people for Covid-19, Whitty defended the UK’s actions.
He said there was “complete surveillance” testing in intensive care, while pneumonia patients were also being tested in hospitals and GPs were testing in the community.
“We do intend to continue to scale up testing,” he said, adding efforts were already “substantial” with more than 44,000 tests conducted.
At the moment tests were only useful for people who were currently sick, but it would be “transformational” if there was a way to find out whether people had previously had it.
That would show what proportion of people can get the disease without any symptoms, he said, adding that Public Health England was “very rapidly” developing such a test.
Vallance said the UK is now looking “more like three weeks” behind Italy, the epicentre of the European outbreak of Covid-19 where there have been more than 1,800 deaths.
Johnson meanwhile said there was “widespread agreement” among G7 countries about what steps need to be taken to lessen the economic impact of the Covid-19 outbreak.
But, asked whether the 1.1% growth target in the Budget is still achievable, he said: “Everybody sees that this is going to be potentially a severe blow for the economy.”
He added: “Everybody can see the risks that this situation imposes, the risk to demand, the risk to supply, but if we get it right and if we get the right response and if we work together then we can ensure that it is a short-term problem because the fundamentals of the UK economy are very strong.
“This is unlike 2008, there isn’t a systemic problem within the economy.”
Johnson added that “if we can get the disease under control in the way that we’re describing, if we can flatten that peak and it starts eventually to decline,” then there is “absolutely no reason why economies worldwide should not come roaring back”.
“The most immediately pressing challenge is the difficulty in keeping schools open with growing numbers of staff having to self-isolate.”
The PM also said the government and parliamentary authorities were working on measures to ensure the Houses of Commons and Lords can stay open while protecting those who were vulnerable.
“The speaker and all parties and the leader of the House of Commons are working together to find a way forward that achieves those objectives - protecting our democracy, ensuring that our parliament stays open but obviously also protecting the groups we need to protect.”
At the same time as the press conference was taking place, leaders of the National Association of Head Teachers (NAHT) and Association of School and College Leaders (ASCL) were meeting with education secretary Gavin Williamson.
The two unions released a joint statement following the meeting, saying schools were struggling to stay open, regardless of the advice.
It is thought, however, the push to keep schools open was designed to keep NHS staff at work.
The statement said: “The most immediately pressing challenge is the difficulty in keeping schools open with growing numbers of staff having to self-isolate.
“It is likely that a number of schools will have to close because there are too few staff available to teach, support and supervise children.”