Everyone in England must stay at home except for permitted reasons during a new coronavirus lockdown expected to last until mid-February, the PM says.
All schools and colleges will close to most pupils and switch to remote learning from Tuesday.
Boris Johnson warned the coming weeks would be the â€œhardest yetâ€ amid surging cases and patient numbers.
He said those in the top four priority groups would be offered a first vaccine dose by the middle of next month.
All care home residents and their carers, everyone aged 70 and over, all frontline health and social care workers, and the clinically extremely vulnerable will be offered one dose of a vaccine by mid-February.
Scotland earlier issued a stay-at-home order and Wales said schools and colleges would shut until 18 January for most pupils.
Schools in Northern Ireland will have an â€œextended period of remote learningâ€, the Stormont Executive said.
Speaking from Downing Street, Mr Johnson told the public to follow the new lockdown rules immediately, before they become law in the early hours of Wednesday.
All the new measures in England will then last until at least the middle of February, he said, as a new more infectious variant of the virus spreads across the UK.
The PM added that he believed the country was entering â€œthe last phase of the struggleâ€.
Hospitals were under â€œmore pressure from Covid than at any time since the start of the pandemicâ€, he said.
And he reiterated the slogan used earlier in the pandemic, urging people to immediately â€œstay at home, protect the NHS and save livesâ€.
At-a-glance: New rules in England
People cannot leave their homes except for certain reasons, like the first lockdown last March
These include essential medical needs, food shopping, exercise and work for those who cannot do so from home
All schools and colleges will close to most pupils from Tuesday with remote learning until February half term
Early years settings such as nurseries will stay open
End-of-year exams will not take place this summer as normal
Elsewhere, university students should not return to campuses and will be taught online
Restaurants can continue to offer food delivery, but takeaway alcohol will be banned
Outdoor sports venues â€“ such as golf courses, tennis courts and outside gyms â€“ must close
But outdoor playgrounds will remain open
Amateur team sports are not allowed, but elite sport such as Premier League football can continue.
On Monday, the UK recorded more than 50,000 new confirmed Covid cases for the seventh day in a row.
A further 58,784 cases and an additional 407 deaths within 28 days of a positive test result were reported, though deaths in Scotland were not recorded.
As of 08:00 GMT, there were 26,626 Covid-19 patients in hospital in England, according to the latest figures.
This is a week-on-week increase of 30%, and a new record high.
Those who are clinically extremely vulnerable will be contacted by letter and should now shield once more, Mr Johnson said.
Support and childcare bubbles will continue under the new measures â€“ and people can meet one person from another household for outdoor exercise.
Communal worship and life events like funerals and weddings can continue, subject to limits on attendance.
While Mr Johnson said end-of-year exams would not take place as normal in the summer, he said alternative arrangements would be announced separately.
The government has published a 22-page document outlining the new rules in detail.
The House of Commons has been recalled to allow MPs to vote on the new restrictions on Wednesday.
Labour leader Sir Keir Starmer said his MPs would â€œsupport the package of measuresâ€, saying â€œweâ€™ve all got to pull together now to make this workâ€.