Speaking at a snap Downing Street press conference on Saturday, the prime minister said strict new measures will remain in place until December 2, with people strongly encouraged to stay at home.
It came as chief scientific officer Patrick Vallance said that if the government took no action the Covid-19 second wave would be “twice as bad or more” than the first.
Johnson said: “The virus is spreading even faster than the reasonable worst case scenario of our scientific advisers.
“Unless we act, we could see deaths in this country running at several thousand a day – a peak of mortality, alas, bigger than the one we saw in April.”
All pubs, restaurants, hospitality venues and leisure facilities will therefore close, but unlike in March schools, colleges and universities will stay open.
In what will be a relief for many workers, the furlough scheme, which sees the government pay 80% of lost wages, will be extended until December 2.
Johnson stressed that during the month-long lockdown, people should only leave their home for education, work, to shop for essential items, recreation outdoors, for medical reasons or to escape harm.
When outdoors, they should only mix with people from their household or with one person from another household, he added.
The PM said he had reached his decision because the overrunning of the NHS would be a “medical and moral disaster, beyond the raw loss of life”.
He said: “Doctors and nurses would be forced to choose which patients to treat, who would get oxygen and who wouldn’t, who would live and who would die.”
He added: “The risk is, for the first time in our lives, the NHS will not be there for us.”
It comes after Johnson chaired an emergency meeting of the Cabinet at Number 10, after the country recorded a spike in coronavirus infections, hospitalisations and deaths. More than 1m coronavirus cases have now been recorded in the UK.
At the meeting, chief medical officer Chris Whitty and Vallance told ministers that every one in every 100 people in England have Covid, compared with one in 2,300 in July and one in 200 at the beginning of October.
The scientists also warned that the NHS would run out of critical care beds by early December, including the beds at the emergency Nightingale hospitals and even if all non-emergency operations were cancelled, if drastic action was not taken quickly.
Whitty warned that the prevalence of coronavirus has been increasing “extremely rapidly” in recent weeks.
Citing Office for National Statistics data, the chief medical officer for England said: “The prevalence of this disease has been going up extremely rapidly over the last few weeks, having been very flat due to the work of everybody in the country over spring and summer.
“And we now have around 50,000 new cases a day and that is rising.”
As during the lockdown in March, takeaways can stay open but only for delivery services.
All other non-essential retail will close and there will be a ban on households mixing indoors in a bid to slow down the Covid-19 second wave.
Manufacturing and construction businesses can stay open but other businesses will close and people must work from home where possible, Johnson said.
After the four weeks, the country is expected to return to the three-tier local lockdown approach, with parts of the country with higher infection rates subject to tougher measures.
International travel will also be banned, unless for work, although Brits abroad will be able to return.
Overnight stays away from home will be permitted only for work purposes and travel within the UK, unless for work, is discouraged.
Places of worship will stay open for private prayer but services will be banned. Funerals will be limited to close family only.
Johnson will speak to MPs on Monday and there will be a vote in parliament on the new measures on Wednesday.
Labour leader Keir Starmer said the government should have introduced a circuit breaker lockdown three weeks ago, when his party backed demands by the Scientific Advisory Group for Emergencies (SAGE) to introduce one.
He said “The government completely rejected that, only to now announce the same thing.
“That delay in introducing restrictions will come at an economic cost and a human cost.
“I’m glad that the government has finally taken this decision - but it should have done so weeks ago.”
Previously, Johnson has clashed with Tory backbenchers who fear the impact new measures may have on the economy.
Wycombe MP Steve Baker, one of the most outspoken proponents of a national lockdown, was seen entering Number 10 on Saturday.
He said he and a data analyst were able to “robustly scrutinise” government plans and that it had been a “serious” conversation.
He said the “prime minister had difficult choices to make”, and added he “would reflect” on what he had heard.
Separately, the “fire break” lockdown introduced in Wales by first minister Mark Drakeford will remains in force until Monday, November 9.
Nicola Sturgeon, meanwhile, has told Scots not to travel to or from England unless it is for “essential purposes”.
The first minister of Scotland said the prevalence of the virus is lower north of the border after stricter measures were introduced in September.
This article originally appeared on HuffPost UK and has been updated.