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...