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".
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
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 - 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 "support the package of measures", saying "we've all got to pull together now to make this work".
Hospital pressure forces PM's hand
Once again it is the threat to the NHS that has forced the hand of ministers.
In England there has been a 50% rise in the number of patients in hospital with Covid since Christmas day.
To put that into context, it equates to 18 hospitals being filled.
Currently around three out of 10 beds are occupied by patients with the disease.
In some hospitals it is more than six in 10.
But what is worrying ministers and NHS leaders is that the number is just going to increase.
In the spring it took nearly three weeks after lockdown for hospital cases to peak.
The last six days have seen in excess of 50,000 new infections confirmed each day across the UK - a number of these infections are next week's hospital admissions.
It is why the UK's chief medical officers were warning there was a "material risk" of some hospitals being overwhelmed if something did not change.
Mr Johnson spoke after UK chief medical officers recommended the Covid threat level be increased to five - its highest level.
Level five means the NHS may soon be unable to handle a further sustained rise in cases, the medical officers said in a joint statement.
NHS Providers, which represents health service trusts, said hospitals were at a "critical point" and that "immediate and decisive action" was needed.
Announcing tougher measures in Scotland, First Minister Nicola Sturgeon said: "It is no exaggeration to say that I am more concerned about the situation we face now than I have been at any time since March last year."
A short-lived return to school
For pupils who returned for their first day of the new term at primary school on Monday, it's turned out to be an extremely short-lived visit.
Boris Johnson's announcement will see primary, secondary and further education colleges closed for at least the next six weeks, except for vulnerable and key workers' children.
It's a much bigger shift in policy than had been anticipated, even a few days ago.
Even the return date will depend on the progress in tackling the virus.
"I hope we can steadily move out of lockdown, reopening schools after the February half term," said the prime minister.
Keeping schools open was the government's most definite of red lines, a few weeks ago they were threatening councils that wanted to close them - but it's now been overtaken by the spiking lines on the Covid infection charts.
Even after the chaos of last year's replacement grades, GCSEs and A-levels are being cancelled again - with a replacement system still to be decided. Vocational exams are to continue.
For parents dreading home schooling, there are plans for it to be better supported this time - with more computer devices available and suggestions that Ofsted inspectors will check what schools are offering.
But there's no escaping that this will feel like another sudden and chaotic change of direction for schools and parents.
Mr Johnson's pledge on vaccinations comes after an 82-year-old retired maintenance manager became the first person in the UK to receive the Oxford-AstraZeneca Covid-19 jab
Some 13.9 million people are among the four priority groups who will receive a vaccine dose by about 15 February, vaccines minister Nadhim Zahawi said.
- NEW YEAR NEW DRAMA: The Serpent: Investigating the disappearance of a young couple in 1975
- FILMS FOR EVERYONE: Check out what films are streaming now on BBC iPlayer
How will you be affected by the latest developments? What questions do you have? Share your experiences by emailing email@example.com.
Please include a contact number if you are willing to speak to a BBC journalist. You can also get in touch in the following ways: