Six million more people in the east and south east of England are to enter tier four on Boxing Day, Health Secretary Matt Hancock has announced.
The places moving into the highest level of restrictions - which include a "stay at home" order - border the areas already in tier four.
A number of areas will also move up into tiers three and two.
Mr Hancock also revealed that another new coronavirus variant from South Africa has been detected in the UK.
He said anyone who had been there in the last two weeks must quarantine immediately.
On Wednesday, a further 39,237 people in the UK tested positive for virus - an all-time high - and there were 744 deaths within 28 days of a positive test, according to government figures.
However, cases were thought to be higher in the UK during the spring peak when testing was much more limited.
The health secretary told the Downing Street briefing the old tiering system was not enough to control the new variant of the virus.
Across the country, cases have risen 57% in the last week, he said, and hospital admissions are at their highest level since mid-April.
The rises have been in places neighbouring areas already in tier four, he said, adding that East Anglia had seen a "significant number" of cases caused by the new fast-spreading variant.
Areas moving to tier four are: Sussex, Oxfordshire, Suffolk, Norfolk, Cambridgeshire, Hampshire, with the exception of the New Forest, and the parts of Essex and Surrey not already in the toughest restrictions.
The additional six million going into tier four takes the total number of people under the toughest restrictions to 24 million, or 43% of England's population. A further 24.8 million will be in tier three.
Mr Hancock said: "This Christmas and the start of 2021 is going to be tough. The new variant makes everything much harder because it spreads so much faster.
"But we mustn't give up now, we know that we can control this virus, we know we can get through this together, we're going to get through it by suppressing the virus until a vaccine can make us safe."(BBC)