Millions of people in the UK are spending a historic Christmas Day apart from loved ones as restrictions limit the number of people who can meet.
Families had originally hoped to be taking advantage of a five-day relaxation of measures from 23 to 27 December, but the emergence of a more transmissible variant of coronavirus meant the rules had to be tightened.
Those living in Tier 4 areas in England are not allowed to meet people outside their own household or support bubble.
Those in Tier 1, 2 and 3 areas are allowed to form a "Christmas bubble" limited to three households on Christmas Day only.
Another six million people are set to enter Tier 4 from a minute past midnight on Boxing Day.
The change will mean 24 million people, representing 43% of the population of England, are living under the strictest measures.
Scotland and Northern Ireland have already announced new lockdowns from Boxing Day, while Wales's tough restrictions will only be eased for Christmas Day before being reimposed.
Thousands of long-distance drivers in Kent will wake up in the cabs of their lorries, uncertain whether they will be allowed to journey home after days of disruption at the Channel border.
The Army has been brought in to help the repatriation operation.
Long queues of lorries formed after French authorities demanded all drivers provide a negative coronavirus test before being allowed to cross.
Some have already spent nearly a week stranded in Kent, although hundreds of lorries have now been given clearance to leave Dover and return to France.
Boris Johnson said he had "never known a Christmas" like this one but said "sacrifices" made this year will keep people alive for next year's festive period.
In a Christmas message filmed by Downing Street, he said: "In most years it's a moment for togetherness and celebration in which the generations are jumbled together in the same household for days on end, pulling crackers and snogging under the mistletoe - you name it.
"And yet this year that is the one type of Christmas we simply cannot afford to have."
Mr Johnson said this Christmas was "not about presents, or turkey, or brandy butter" but about hope in the form of the COVID-19 vaccines being developed.
He added: "It's thanks to the efforts of wise men and wise women in the east and elsewhere, we have a vaccine and we know that we are going to succeed in beating coronavirus, and that these privations that we're going through are temporary and we know that next year really will be better.
"We know there will be people alive next Christmas, people we love, alive next Christmas precisely because we made the sacrifice and didn't celebrate as normal this Christmas."
A further 39,036 coronavirus cases were confirmed in the UK on Christmas Eve - the second highest daily total to date, according to government figures.
The figure brought the total number of confirmed cases since the pandemic began to 2,188,587.
Another 574 deaths were confirmed on Christmas Eve - bringing the total in the UK to 69,625 since the pandemic began.
It comes as more than 600,000 people in the UK have received the first dose of the Pfizer/BioNTech vaccine.
The vaccines have been administered to care home residents, those aged 80 and over, and health and social care staff through more than 500 vaccination sites across the UK.
The groups have been prioritised under government guidance in line with advice from the the Joint Committee on Vaccination and Immunisation (JCVI).