• Johnson said England's health system is at risk of becoming overwhelmed in 21 days without a national lockdown.
• People can only leave their homes to shop for essentials, work if they can't from home, exercise, go to the doctor's and escape domestic abuse, he said in an announcement Monday evening.
• Primary schools, secondary schools and colleges will move to remote learning Tuesday.

British Prime Minister Boris Johnson said England is adopting a national lockdown that he hopes will be tough enough to contain a new, highly contagious variant of Covid-19.

People can only leave their homes to shop for essentials, work if they can't from home, exercise, go to the doctor's and escape domestic abuse, he said in an announcement Monday evening. Primary schools, secondary schools and colleges will also move to remote learning Tuesday, except in rare cases, he said.

"I completely understand the inconvenience and distress this change will cause millions of people and parents up and down the country," Johnson said. "The problem isn't that schools are unsafe for children ... the problem is that schools may act as vectors of transmission, causing the virus to spread between households."

The U.K.'s chief medical officers recommended the country move to alert level 5, meaning that if the country doesn't take action the National Health Service capacity "may be overwhelmed in 21 days," Johnson said.

The changes come as the U.K. grapples with a more transmissible variant of Covid-19. To date, the country has recorded over 2.6 million cases of coronavirus and more than 75,000 related deaths, according to data compiled by Johns Hopkins University.

On Monday, the U.K. recorded 58,784 new cases, and has now reported more than 50,000 new coronavirus cases for seven days in a row.

"The number of deaths is up by 20% over the last week and will sadly rise further. ... With most of the country already under extreme measures, it's clear that we need to do more together to bring this new variant under control while our vaccines are rolled out," Johnson said, noting that the mutated strain is estimated to be 50% to 70% more contagious.

Johnson warned earlier Monday that the U.K. had "tough, tough weeks to come" and there was "no question" tougher measures would be implemented.

Ahead of the announcement, more than three-quarters of England were living under the toughest level of restrictions.

On Monday afternoon, Scotland's leader, Nicola Sturgeon, announced a new stay-at-home order for the country's citizens from midnight. Schools in Scotland will remain closed until the beginning of February.

Keir Starmer, leader of the U.K.'s main opposition Labour Party, tweeted Sunday that Johnson "must put national restrictions in place within the next 24 hours."

Coronavirus vaccines are the only bright spot in a pandemic that continues to rage across the U.K. and much of the West. On Monday, the U.K. began its rollout of the Oxford-AstraZeneca vaccine after starting to deploy the Pfizer-BioNTech shot in December.

"There's one huge difference compared to last year. We're now rolling out the biggest vaccination program in our history," Johnson said.

If things go well, Johnson said everyone in the top four priority groups should get the first shot of the two-dose vaccines by the middle of February. That includes residents in a care home and their caregivers, everyone over the age of 70, all front-line health and social workers as well as everyone who is clinically vulnerable, he said.

"If we succeed in vaccinating all those groups, we will have removed huge numbers of people from the path of the virus. And of course that will eventually enable us to lift many of the restrictions we have endured for so long," he said.

The country could consider reopening schools after the February half term if the vaccine rollout continues to go well, deaths start to fall and if "everyone plays their part by following the rules."

He said the coming weeks will be the hardest yet, "but I really do believe that we're entering the last phase of the struggle because with every jab that goes into our arms, we are tilting the odds against Covid and in favor of the British people," he said. "I know how tough this is and I know how frustrated you are and I know that you've had more than enough of government guidance about defeating this virus. But now, more than ever, we must pull together."

The U.K. government has decided to implement a 12-week delay between the first and second doses of both the Pfizer-BioNTech and Oxford-AstraZeneca coronavirus vaccines, in a bid to cover as much of the population as possible.

The U.K.'s independent Scientific Advisory Group for Emergencies said Sunday that it endorsed the move, with conditions, however the British Medical Association has criticized the U.K.'s decision to delay second doses.