The planned relaxation of coronavirus rules for Christmas has been scrapped for a large part of southeast England, and cut to one day for rest of the region, Xinhua news agency quoted Johnson said at a virtual press conference at Downing Street on Saturday evening.
From Sunday morning, London, the South East and East of England will move into Tier Four restrictions, which will be broadly similar to national restrictions introduced in England in November, he said.
Under the new tougher measures, residents in those areas must stay at home, with limited exemptions.
People are also urged to work from home when they can, and should not enter or leave tier four areas.
Meanwhile, non-essential retails and indoors gyms must close, the Prime Minister said.
The restrictions will last for two weeks and will be reviewed on December 30, Johnson said.
He said the previous plan to allow up to three households to meet during five days around Christmas will have to be scrapped.
“We cannot continue with Christmas as planned,” Johnson said.
People should not to travel into a Tier Four area, though support bubbles remain unaffected as do the exemptions for separated parents and their children.
For those in Tiers One, Two and Three, rules allowing up to three households to meet will now be limited to Christmas Day only, while in Tier Four, people should not mix with anyone outside their own household, apart from support bubbles.
The Prime Minister said the latest infections in parts of England are “being driven by the new variant of the virus”.
“New variant may be up to 70 per cent more transmissible,” he said.
The new restrictions aim to slow the spread of the mutant virus to other parts of Britain.
The British government has informed the World Health Organisation about its findings so far on the new variant of the virus, local media reported.
Joining Johnson for the press briefing, the government’s Chief Scientific Adviser Patrick Vallance said the new variant was first seen in mid-September in London and Kent.
By December, the new variant had become the “dominant” in London, said Vallance.
By the week ending December 9, the new variant accounted for 62 per cent in London, 59 per cent in eastern England, and 43 per cent in the South East, according to Vallance.
The latest announcement came after Johnson called a meeting of Ministers on Saturday afternoon to discuss the crisis.
England was previously under a three-tier system.
London and parts of Essex and Hertfordshire had just moved into Tier Three on December 16, while some other areas in east and southeast of England entered Tier Three from December 19 following a rapid rise in infections in the area.
Also on Saturday, Welsh First Minister Mark Drakeford announced that the whole of Wales will be placed under lockdown from Sunday with Christmas plans cancelled for all but one day.
“The situation is incredibly serious. I cannot overstate this,” Drakeford said.
This comes as the overall coronavirus casload in the UK has increased to 2,010,077, while the death toll stood at 67,177.