England is being placed into a new national lockdown from Thursday, with pubs, leisure facilities, non-essential shops and other businesses being ordered to close, Boris Johnson has announced.
The lockdown will last four weeks, starting at midnight on Thursday and ending on December 2.
Mr Johnson warned there will be "several thousand" deaths a day, "far" more than the first peak of coronavirus, if action isn't taken.
Without further action, the NHS will breach its fixed and surged bed capacity -including beds in Nightingale hospitals - by the first week of December even if planned operations are cancelled, according to modelling by the Scientific Pandemic Influenza Group on Modelling (SPI-M), which advises Government scientists, Mr Johnson said.
He said NHS capacity would be impacted so much that doctors and nurses would be forced to decide "who would live and who would die".
Apologising to businesses, the PM said the government will extend furlough payments at 80% for the duration of the new national lockdown measures in England.
He warned that "Christmas is going to different this year, perhaps very different" but added that by taking action now he hoped that families could be together.
All indoor household mixing will be banned and people will only be allowed to meet outdoors in a public space with one other person from outside their household - meetings in private gardens will also be banned.
Stay at home advice is returning, meaning people will only be able to venture outdoors for education, work - if they cannot work from home - exercise and recreation.
Other exceptions include leaving the house for medical reasons and to escape injury or harm.
Support bubbles are allowed to continue. Elite sport - such as football's Premier League - will also be allowed to play on.
As such, Mr Johnson insisted the new lockdown is not the same as the "full scale lockdown" of the spring. He said the new measures are "less restrictive".
Businesses being ordered to close include, pubs, bars and restaurants - except for takeaway and delivery.
Non-essential shops will close, as will leisure facilities like gyms and businesses providing services such as hairdressing or beauty treatments.
Education settings, such as schools, colleges, universities and nurseries will be allowed to remain open.
Places of worship will only be allowed to open for private prayer, meaning services will be banned.
Funerals will still be allowed to take place, with a maximum of 30 people and linked events such as stone settings and ash scatterings can continue with a maximum of 15 people.
Weddings will not be permitted to take place except in "exceptional circumstances".
International travel out of the country, except for essential work reasons, will be banned and anyone returning to England will be forced to quarantine for two weeks.
Use of public transport will still be allowed for those going to work but they are being encouraged to walk or cycle where possible.
Labour leader Sir Keir Starmer - who argued for a circuit-breaker a few weeks ago - said he supported this move:
“Everybody is concerned about the rise in infections, the hospital admissions and tragically the number of deaths. That’s why three weeks ago, I called for circuit-break," he said.
“The Government completely rejected that only now to announce the self-same thing.
“Alas the delay now will cost, the lockdown will be longer, it’ll be harder and there’s a human cost which will be very, very real.
First Minister of Scotland Nicola Sturgeon said she will "carefully consider" the impact of Mr Johnson's announcement, but added "we will continue to take decisions that reflect circumstances in Scotland".
She earlier urged Scots to avoid travelling to England.
The changes are being made to preserve the capacity of the NHS to carry out essential care.
Government forecasters believe the NHS will surpass all bed capacity by December 4 if the spread of coronavirus is not tackled.
Chief Medical Officer Professor Chris Whitty warned that the prevalence of Covid has been increasing "extremely rapidly" in recent weeks.
"We now have around 50,000 new cases a day and that is rising," he said.
Chief Scientific Adviser Patrick Vallance said: "There’s the potential for this to be twice as bad, or more, as the first wave."Discussing NHS bed use in England, Professor Whitty said: "Currently only in the North West is this coming close to the peak that we previously had, but it is increasing in every area. "And if we do nothing, the inevitable result is these numbers will go up and they will eventually exceed the peak that we saw in the spring of this year."
Mr Johnson said "rapid turnaround" tests will be used to reduce the prevalence of the virus, and tests of "whole cities" and the Army has been drafted in to assist with roll out within a "matter of days".
At the end of lockdown the government plans to return restrictions to a regional approach, meaning areas will again be designated Tier 1, 2 or 3.
Mr Johnson will update MPs on the changes in Parliament on Monday, and a vote on the new lockdown will be held on Wednesday.