The planned relaxation of Covid rules for Christmas has been scrapped for large parts of south-east England and cut to just Christmas Day for the rest of England, Scotland and Wales.
From midnight, a new tier four will be introduced in areas including London, Kent, Essex and Bedfordshire.
Those in tier four cannot mix indoors with anyone not from their household.
Elsewhere in England, Scotland and Wales, relaxed indoor mixing rules are cut from five days to Christmas Day.
Prime Minister Boris Johnson announced the changes for England at a Downing Street briefing after scientists said a new coronavirus variant was spreading more rapidly.
Tier-four restrictions - similar to England's second national lockdown - will apply in all areas in the South East currently in tier three, covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It will also apply 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).
In Scotland, Covid restrictions will only be relaxed on Christmas Day, with mainland Scotland being placed under the tightest restrictions from Boxing Day.
