The United Kingdom has the world's sixth-highest official coronavirus death toll - 75,024 - and the number of new infections is soaring across the country.
Scottish First Minister Nicola Sturgeon said on Monday the new variant accounted for nearly half of new cases in Scotland and is 70% more transmissible.
Scots, she said, would be legally required to stay at home for January from midnight. Schools will close for all but the children of essential workers.
"I am more concerned about the situation that we face now than I have been at any time since March," Sturgeon told the Scottish parliament.
"As a result of this new variant, (the virus) has just learned to run much faster, and has most definitely picked up pace in the past couple of weeks."
Visiting a hospital to see the first people receive the vaccine made by Oxford University and AstraZeneca, Johnson said the country faced "tough, tough" weeks to come.
"If you look at the numbers, there's no question that we're going to have to take tougher measures and we'll be announcing those in due course," Johnson said. "We've got the virus really surging."
England, Scotland, Wales and Northern Ireland implement their own Covid-19 responses though they are trying to coordinate more across the United Kingdom.
MP ARRESTED OVER COVID RULES
Scottish police have arrested Margaret Ferrier, a member of Britain's parliament, for allegedly breaking Covid-19 rules when she made a long train journey last year after testing positive for the virus.
"We can confirm that officers today arrested and charged a 60-year-old woman in connection with alleged culpable and reckless conduct," a Police Scotland spokeswoman said on Monday.
"This follows a thorough investigation by Police Scotland into an alleged breach of coronavirus regulations between 26 and 29 September 2020. A report will be sent to the procurator fiscal and we are unable to comment further."
Ferrier learned that her test was positive at the end of September after she had spoken in Britain's parliament at Westminster. She has apologised for then taking the train more than 400 miles back to Scotland.
Her Scottish National Party (SNP) suspended her shortly afterwards.
It is mandatory in England for people to self-isolate if they test positive for the novel coronavirus.
