Boris Johnson has announced that England is facing a third national lockdown, instructing everyone to “stay at home".
All schools will also close from tomorrow.
The prime minister made the call in a televised address on Monday evening, in which he said the “weeks ahead will be hardest yet”.
His announcement follows Scotland's First Minister Nicola Sturgeon, who announced a lockdown that will begin at midnight.
A scientific adviser to the government has said the latest shutdown was "inevitable" after Christmas Day mixing was allowed and a new, more transmissible variant of the virus, was identified.
People will be allowed to leave their homes for limited reasons like shopping for essentials, exercise, and medical assistance.
Mr Johnson also said people could still leave home "to escape domestic abuse". In the early weeks of the pandemic, isolation and lockdown resulted in a high rise in abuse cases, with victims having no way to escape their perpetrators.
Outdoor sports venues will have to close. But unlike spring's lockdown, nurseries will not be shuttered, elite sports can go ahead, and places of worship will remain open on the basis that attendees adhere to social distancing rules.
Meanwhile, minutes released from a meeting of the Scientific Advisory Group for Emergencies (SAGE) revealed ministers were urged two weeks ago to shut schools due to rising coronavirus infections.
However, the prime minister was still insisting on Sunday that he had "no doubt" that schools were "safe".
SAGE told the government on 22 December it was "highly unlikely" that allowing schools to open would keep the R number below 1, meaning the number of cases would continue to grow.
The UK's new daily Covid-19 cases soared above 50,000 for nearly a week, and hospitalisations have exceeded April's peak.
According to the prime minister, there were 30% more Covid-19 patients in hospitals in England on Monday than a week earlier.
The health minister Matt Hancock said the country would move to the highest Covid-19 alert level, reflecting concern that the rising number of people in hospitals could overwhelm the National Health Service (NHS) in coming weeks.
Despite scrapping plans to relax restrictions for five days over the festive period, millions of people across England were allowed to mix with other households on Christmas day.
Mr Johnson announced on 19 December that up to three households could gather for just one day, apart from those in areas under Tier 4 restrictions.
New coronavirus variant
The new variant of coronavirus in the UK has been blamed for the rise in infections around the country.
A rise in cases linked to the variant was identified as far back as October and ministers had been warned about its increased transmissibility in November.
The government is hoping to rollout the
Covid-19 vaccine as soon as possible. However, there is still a long way to go before the millions in the top priority groups, mentioned by Johnson are vaccinated, including people "over the age of 70, all frontline health workers and everyone who is clinically vulnerable.
The prime minister has said that these groups are expected to have been offered their first doses by mid-February. However, since the beginning of December, when the UK became the first in the world to administer the clinically approved Pfizer/BionTech vaccaine, more than 1 million people have received the immunisation.
Students in the UK were due to return to school from the Christmas break on Monday. But less than a week ago, the government announced the return to school would be delayed by two weeks for almost all high-school pupils and some primary school children, with classes to take place remotely.
Major exams cancelled again
Meanwhile, GCSE and A Level students in England will not be sitting exams this summer once again as a result of the new lockdown measures that are likely to go on until mid-February.
Ministers are said to be working with the exams regulator Ofqual to find a system for awarding grades that "reflects the hard work" of pupils across the country.