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 “hardest yet” 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.
Schools in Northern Ireland will have an “extended period of remote learning”, 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 “the last phase of the struggle”.
Hospitals were under “more pressure from Covid than at any time since the start of the pandemic”, he said.
And he reiterated the slogan used earlier in the pandemic, urging people to immediately “stay at home, protect the NHS and save lives”.
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.
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 – such as golf courses, tennis courts and outside gyms – must close
*But outdoor playgrounds will remain open
*Amateur team sports are not allowed, but elite sport such as Premier League football can continue.