The public will only be allowed to leave home for one of four reasons, with the police poised to issue fines and break up gatherings if rules are ignored.
Weddings, baptisms and other gatherings are now banned, and all shops not selling essential items have been ordered to close, along with places of worship and playgrounds and outdoor gyms.
Addressing the nation from inside Number 10 Downing Street tonight Boris Johnson said: “No Prime Minister wants to enact measures like this.”
He said: “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.”
People will now only be allowed out of their homes to buy essential supplies, to exercise once a day, for medical visits or to provide care or to help a vulnerable person, or to travel to and from work.
However employees should only go to work where it is absolutely necessary.
Mr Johnson said: “That’s all - these are the only reasons you should leave your home.
“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.”
It comes after the World Health Organisation warned the pandemic is “accelerating”. The global health body said it has now received more than 300,000 reports of Covid-19.
The first 100,000 cases were reported in the first 67 days of the virus, the second 100,000 cases came 11 days later and the next 100,000 cases just came four days after that, it said.
In the UK the death toll hit 335 today, six of these deaths have been in Yorkshire. There were 281 cases of coronavirus identified in the region, compared to more than 5,000 country-wide.
The new UK measures come into effect tonight and will be reviewed in three weeks.
Mr Johnson said: “There are just no easy options. The way ahead is hard, and it is still true that many lives will sadly be lost."
The draconian measures come after reports those in Government were furious as photographs emerged over the weekend of people floating social distancing rules.
Health Secretary Matt Hancock said earlier he did not know why some people were ignoring the Government's advice.
"It's very selfish," he said. "The NHS is doing everything it can and preparing for the spread of this virus.
"If people go within two metres of others who they don't live with then they're helping to spread the virus - and the consequences of that costs lives and it means that, for everyone, this will go on for longer."
Labour leader Jeremy Corbyn said: "The Prime Minister is right to call for people to stay at home, protect our NHS and save lives.
"This is the right response to the coronavirus pandemic, and one we have been calling for.
"There now needs to be clear guidance to employers and workers about which workplaces should close – and the Government must close the loopholes to give security to all workers, including the self-employed, as well as renters and mortgage holders.
"We welcome these moves and will be working to ensure everybody has the protection and security they need.”
In his address Mr Johnson added that there is also a "clear way through" this crisis.
He said: "Day by day we are strengthening our amazing NHS with 7500 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.
"I want to thank everyone who is working flat out to beat the virus.
"Everyone from the supermarket staff to the transport workers to the carers to the nurses and doctors on the frontline.
"But 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."