Boris Johnson has faced claims that he ignored scientific advice in the run-up to imposing a third national lockdown in England.
The prime minister has instructed people to "stay at home" following the surge in coronavirus infections, closing all non-essential shops and ordering schools to move to remote learning.
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.
But what did experts tell the government behind the scenes in the days and weeks before lockdown was announced, and have ministers been "following the science" as they have repeatedly claimed? Sky News takes a look.
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.
Despite this, 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 coronavirus reproduction rate - or R number - below 1, meaning the number of cases would continue to grow.
It followed the emergence of the new variant of coronavirus in the UK.
"R would be lower with schools closed, with closure of secondary schools likely to have a greater effect than closure of primary schools," SAGE said.
But on Sunday, Mr Johnson said: "There is no doubt in my mind that schools are safe, and that education is a priority."
He added: "The benefits of education are so huge, overwhelmingly we want to keep our young people in education."
On Monday, most primary schools in England reopened after the Christmas break, but that evening the prime minister ordered schools to move to remote learning as part of the national lockdown.
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.
A month earlier, SAGE had warned the government that "any relaxation" of coronavirus restrictions over Christmas would increase infections "potentially by a large amount".
In a document called "notes on festive period", which was written from 15-18 November, SAGE said the prevalence of COVID-19 "could easily double" over the festive period with "substantial mixing" of people.
"A parallel can be drawn, albeit on a different scale, between the return of students in autumn and people from different households mixing intensively over Christmas," SAGE said.
Grilling the prime minister on his plan to introduce Christmas bubbles, Labour leader Sir Keir Starmer said on 16 December: "Here we go again... ignoring medical advice. And we know where that leads because we've seen what happened in the last nine months."
New coronavirus variant
The new variant of coronavirus in the UK has been blamed for the rise in infections around the country.
In his TV address on Monday night, Mr Johnson said the variant was judged to be between 50% and 70% more transmissible and was spreading at a "frustrating and alarming" speed.
But a rise in cases linked to the variant was identified as far back as November and ministers had been warned about its increased transmissibility last month.
On 18 December, the government's New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) told ministers it had "moderate confidence" that the new variant "demonstrates a substantial increase in transmissibility compared to other variants".
Health Secretary Matt Hancock revealed on 14 December that the variant had been identified in the UK and it was believed to be causing a faster spread of the virus in the South East.
But the government has said the increase in cases linked to the variant "first came to light in late November when Public Health England was investigating why infection rates in Kent were not falling despite national restrictions".
Evidence suggested it emerged in September and then "circulated at very low levels in the population until mid-November", the government added.
What do the experts say now?
Dr Julian W Tang, clinical virologist at the University of Leicester, said "we cannot really blame the new variant virus for all of this" after the national lockdown was introduced.
He said: "We sort of predicted the likely need for this post-Xmas/New Year lockdown because we knew that many people would still try to meet up over the festive season.
"Yet this brief freedom will have long-term costs for business and education - and people's health, both COVID-19 and non COVID-19 related.
"It's amazing that we seem to be making the same mistakes over and over again - with increasing loss of life.
"The vaccine is probably the only way out of this for the UK, as the various tiered social distancing measures are clearly not working well enough here."
SAGE member Prof Calum Semple said a national lockdown was "inevitable" after Christmas Day mixing was allowed and the new variant of coronavirus was identified.
He told Sky News he had seen "nurses in tears, doctors with their heads in their hands" as they struggle to cope with the increase in COVID-19 patients in hospitals.
"We're only now seeing the start of the price we have to pay for Christmas Eve and Christmas Day mixing," Prof Semple said.
"With that and the new variant it was inevitable we were going to have to hit a hard lockdown at this stage."
His comments were echoed by Professor Neil Ferguson, a member of NERVTAG, who said: "The new variant made these measures inevitable and necessary.
"The next few weeks will show whether they are enough to suppress the much more transmissible new variant which is now predominating in the country."