Boris Johnson has announced strict coronavirus restrictions for England over Christmas, with London, the South-East and East also placed under a new tier 4.
It comes amid fears a new variant of Covid-19 is spreading more rapidly across parts of the country.
Speaking at a Downing Street press conference on Saturday, the prime minister said the variant could be “up to 70% more transmissible” than the original variant.
People living in tier 4 areas will be banned entirely from mixing households over Christmas.
In all other areas of England, Christmas bubbles will only be permitted on Christmas Day itself.
The government had previously told people three households would be allowed to mix for five days between December 23 and December 27.
Under the new tier 4 rules, which came into force on Sunday, people have been ordered to stay at home by law and non-essential retail will shut.
In tier 4 areas, one person is allowed to meet one other person not in their household and it must be outside. Travel is be banned into and out of tier 4 areas.
The new rules will be reviewed after two weeks.
Tier 4 restrictions apply in all previous tier 3 areas in the South-East, covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It also applies in London (all 32 boroughs and the City of London) and the East of England (Bedford, Central Bedford, Milton Keynes, Luton, Peterborough, Hertfordshire, Essex (excluding Colchester, Uttlesford and Tendring).
Chris Whitty, England’s chief medical officer, has said there is no evidence the new variant of the virus is more deadly or will affect the vaccine, but it can spread more quickly.
Johnson, who just three days ago said preventing people spending Christmas with friends and family would be “inhuman”, said he was taking just that action now “with a very heavy heart”.
“I know how much emotion people invest in this time of year, and how important it is, for instance, for grandparents to see their grandchildren, for families to be together,” he said. “So I know how disappointing this will be.
“But we have said throughout this pandemic that we must and we will be guided by the science. When the science changes, we must change our response.”
Exemptions to the stay at home message which applied in the November lockdown, also apply in the new tier 4 – including support bubbles, childcare bubbles and children whose parents are separated.
People are allowed to travel for education, childcare and to go to work if they cannot work from home and are permitted unlimited outdoor exercise.
In Wales, level four lockdown restrictions will be brought forward to apply across the country from Sunday and festive bubbles can be formed for Christmas Day only, first minister Mark Drakeford also announced on Saturday.