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.
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 “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”.