Boris Johnson announced a new Tier 4 on Saturday December 19, with tighter restrictions for London and the South East, while cutting back on the planned relaxation of the rules over Christmas.
Before the rapid rise of the new Covid strain, all four UK nations had agreed that up to three households could meet in a private setting between Dec 23 and 27 (between Dec 22 to 28 in Northern Ireland) in a temporary break from the current three-tier system.
However, the Prime Minister was forced to hold emergency talks with Cabinet ministers, as scientists confirmed that the new variant of Covid-19 was spreading rapidly in some areas.
He said that "with a heavy heart", the three household rule will now only apply to Dec 25, rather than the whole period of Dec 23 to 27.
Meanwhile, those in Tier 4 areas cannot mix with any other households over Christmas, but support bubbles can continue for those at particular risk of isolation or loneliness.
The rules came into effect on Sunday morning and will next be reviewed on Dec 30.
Mr Johnson made the announcement at a press conference with Prof Chris Whitty, the chief medical officer for England, and the Government's chief scientific adviser, Sir Patrick Vallance, on Saturday afternoon.
"I know how much emotion people invest in this time of year and how important it is for grandparents to see their grandchildren, and families to be together, so I know how disappointing this will be," he said.
"Yes Christmas this year will be different but we must be realistic. We're sacrificing the chance to see our loved-ones so we have a better chance of protecting their lives and seeing them at future Christmases."
The latest development came after Mr Johnson refused to rule out a third national lockdown for England - a move he had previously been deeply reluctant to contemplate.
All the latest updates for the UK rules over Christmas are below.
Christmas rules in England
In all four nations, you can meet another household on Dec 25 only.
In a joint statement, the Cabinet Office previously announced the leaders of England, Wales, Scotland and Northern Ireland had “endorsed a shared objective of facilitating some limited additional household bubbling for a small number of days”.
They “emphasised that the public will be advised to remain cautious, and that wherever possible people should avoid travelling and minimise social contact”.
However, this has now been modified to apply to just Dec 25, rather than Dec 23 to 27.
But what do these restrictions mean?
Three households could meet indoors, meaning families will be able to have both sets of grandparents to stay. Under tier 4, however, you cannot meet other people indoors unless you live with them or they are part of your existing support bubble.
Couples can join different bubbles - so if a husband and wife want to see their own families, each can do so, although they cannot switch between these. Children of divorced parents will be allowed to split their time between two bubbles.
While you can form a bubble with up to three households, these bubbles are not permitted inside pubs or any other hospitality venues.
Access to pubs and restaurants will still depend on the rules of your local tier system.
Grottos are allowed to open across all tiers, except tier 4, new government guidance has confirmed, but sitting on Santa’s lap is banned.
Venues must put in place appropriate Covid-secure measures, and families are required to maintain social distancing from Father Christmas.
Door-to-door carol singing is permitted as long as groups are outdoors and keep apart from each other.
Those in Tier 3 are not able to attend school nativity plays and will have to live stream or watch a recording instead. Performances need to be within existing school bubbles, with no mixing across groups.
In Tiers 1 and 2, audiences can attend “subject to appropriate safeguards being in place”.
Over 65s in care homes will not be able to go home for Christmas.
University students travelling back home at the end of the term will count as part of their parents’ household.
People flying home from red-listed countries will still have to quarantine.
Government guidelines also state that people in a bubble should:
Limit unessential contact beyond your immediate household at least five days before you mix with other homes in your bubble
Keep your bubble as small as possible
Only exist in one Christmas bubble
Not change your Christmas bubble
Meet your Christmas bubble in private homes or your garden, places of worship, or public outdoor spaces
Only mix with people outside your Christmas bubble outside your home following the rules in the tier you are meeting in
Not meet socially with family or friends that you do not live within your home or garden unless they are part of your Christmas bubble
Christmas rules in Scotland
A majority of these rules are the same in Scotland after the four countries agreed to implement the same restrictions over the festive period.
However, despite this united agreement, the Scottish Government have set a cap of eight people over 12 years old for Christmas gatherings. They also emphasised that households should remain two metres apart while inside.
Scottish First Minister, Nicola Sturgeon, fuelled rumours that the four nations may diverge their united Christmas plans, as she announced case levels in Scotland, saying, we are, “at the moment lower than in other nations” but stressed the need to consider the “evolving situation”.
The Scottish Government has also announced that people living in the same house can’t join different bubbles. They also emphasised that households should remain two metres apart while inside. Scotland will not consider existing bubbles between two households as one, whereas nations south of the border will.
Though, Ms Sturgeon has urged Scots to stay in their own homes at Christmas.
If people feel it is "essential" to meet with others indoors, she said this should be for one day only and not overnight.
The First Minister also suggested families should give each other vouchers for Christmas, for health and economic reasons, and warning she would tighten Christmas restrictions around the edges.
Christmas rules in Wales
Once again, the majority of rules set by the Government will also be rolled out across Wales.
This comes after Mr Drakeford, along with the other leaders of the three nations, agreed to continue with the easing of restrictions during Christmas, but have said they will communicate a more robust message warning people of the dangers.
On December 16, Mr Drakeford announced that Wales would go into full lockdown from December 28, after a "sustained rise in coronavirus"
The Welsh leader shared a "sustained rise in coronavirus" meant that the country would have to move into its highest level of restrictions - alert level four.
This means that only two households are allowed to mix over the festive period, and all hospitality premises will have to close their doors 6pm on Christmas Day.
This harsh limitation will be made into law, the Welsh Government has confirmed.
This news comes only hours after the First Minister stated that lowering the number of homes that can form a Christmas bubble from three to two was guidance rather than a requirement.
Christmas rules in Northern Ireland
As part of the united Christmas agreement, Northern Ireland negotiated a seven-day suspension of travel rules, which allows people to catch flights or ferries to Britain.
The Northern Ireland Executive decided last week to impose a six-week lockdown from December 26.
But over Christmas people from up to three households will be allowed to congregate together in "social bubbles" between December 23 and 27, unlike the rest of the UK.
The first week of the measures will see the toughest lockdown yet in Northern Ireland, with a form of curfew in operation from 8pm, shops closed from that time and all indoor and outdoor gatherings prohibited until 6am.
Non-essential retail will close throughout the six weeks, as will close contact services. Hospitality outlets will be limited to takeaway services.
Organised sport will also be banned, with elite sport included in the prohibition for the first week.
The latest rules in Northern Ireland are here.
Can I travel between tiers over Christmas?
A cross-border travel ban between Scotland and England has been in place in recent weeks but it was due to be lifted for five days from Dec 23.
Nicola Sturgeon has now extended that ban over Christmas, saying she was "very, very sorry" but it was needed to prevent any more of the new strain entering the country.
Wales, too, has re-entered a national lockdown that bans incoming and outgoing travel.
As for individual countries, each nation has different regulations once travellers arrive at their destination. In England, you should follow the rules of the tier in the area you are visiting.
Within a Tier 4 area people can only leave home for work, education or the limited number of other legally permitted reasons.
Residents are encouraged to keep their travel local, reduce their number of journeys and avoid leaving their village, town or city area.
Walking and cycling is also recommended, with people advised to avoid public transport at busy times.
You should not leave a Tier 4 area unless for permitted reasons, such as work, education, caring responsibilities, to visit a support bubble or for medical reasons, and someone from a Tier 1, 2 or 3 area should not enter a Tier 4 zone except for the same reasons.
In Scotland, you should also follow the travel advice for the level you are in during the relaxation period. For example, visitors staying in a level 3 area cannot go on an outing to a level 2 area.
Meanwhile, Northern Ireland is negotiating an agreement with Ireland. This suggests travel rules may be the same in every part of the British Isles.
Can I travel abroad over Christmas?
Travel, both domestic and international, is off the cards for the majority of the UK until at least December 30.
Those in Tier 4 are not permitted to leave the country apart from "limited exceptions", such as for work purposes, the prime minister said.
Mr Johnson urged people to "carefully consider whether they need to travel abroad." No outright ban was indicated, as per the first lockdown.
However, the British Government has notified the World Health Organisation about the new strain so that other countries could prevent Britons from travelling there if they wish.
The first to do so was the Dutch government, which banned all passenger flights from Britain until January 1 after finding a case in the Netherlands of the new coronavirus strain.
In the UK, all tiers are being advised to "stay local" by the Prime Minister. In Tier 3, residents were also advised not to travel abroad or to other areas of the UK.
For those who are allowed to leave the country, the green list of travel corridor countries, which Britons can visit without needing to self-isolate upon return, is shrinking, but quarantine will be lifted for those who test negative for Covid-19 on the fifth day of isolation.
Travelling against official government advice is not illegal, but most tour operators will not offer trips to destinations which the Foreign Office (FCO) deems unsafe. In a pandemic, that's just about everywhere - and it does make travel insurance complicated.
If you choose to visit a country to which the FCO advises against travel without invalidating your insurance - here's what you need to know.