"We are not confident that the NHS (National Health Service) can handle a further sustained rise in cases and without further action there is a material risk of the NHS in several areas being overwhelmed over the next 21 days", the chief medical officers of England, Wales, Scotland and Northern Ireland, and the medical director of NHS England said in a statement.
Beginning Tuesday, people in Scotland will be required to stay at home except for essential reasons, to help ease the pressure on hospitals and intensive care units, Sturgeon said.
So we should remain cautious about the timetable ahead.
On Monday Britain became the first country to vaccinate its population with Oxford University and AstraZeneca's COVID-19 shot and Mr Johnson is set to address the nation on Monday evening.
Earlier on Monday, the chief medical officers of all four United Kingdom nations recommended the nationwide COVID alert level move from level 4 to level 5 - the first time it has hit its most severe level.
A double-dip recession is looking increasingly inevitable as a result of the new lockdown.
The new lockdown, expected to be in place until the middle of February, will come into effect in law, and parliament is expected to consider it on Wednesday.
The government now aims to rush out a vaccination program, reaching 13.9 million vulnerable people and carers by mid-February so restrictions can begin to be lifted.
In a televised address, the PM urged people to follow the rules immediately amid surging cases and patient numbers.
Johnson said people must stay at home again, as they were ordered to do so in the first wave of the pandemic in March, this time because the new virus variant was spreading in a "frustrating and alarming" way. People were told to work from home unless it's impossible to do so, and leave home only for essential trips.
Early years settings will remain open.
Boris Johnson's father seeks French citizenship as United Kingdom leaves EU
Big Ben chimed at 11pm, when the Prime Minister is understood to have marked the occasion with his family in Downing Street. The 1,246-page trade agreement, signed off on December 24, created a free trade area with zero tariffs or quotas.
Primary schools in London are closed for at least two weeks, and many high school students in England have had their return date pushed back until later in January.
With the new lockdown, all non-essential shops will be shut across England, along with bars, restaurants and other hospitality and entertainment venues. Food delivery will be allowed, but not takeaway alcohol.
Elite sport, including Premier League soccer, will continue.
The clinically vulnerable must shield again and will soon receive a letter. Most recently, Johnson was forced to curtail plans to relax social-distancing rules over Christmas.
"The spread of the new variant of COVID-19 has led to rapidly escalating case numbers across the country".
Britain - one of the worst-affected countries by the global health crisis, with over 75,000 deaths - has been pinning its hopes on the mass vaccination drive.
"I am so pleased to be getting the COVID-19 vaccine today and really proud that it is one that was invented in Oxford", said Pinker, a retired maintenance manager, just a few hundred meters from where the vaccine was developed.
But even with the vaccines being rolled out, the number of Covid-19 cases and deaths keep rising. Now the country has recorded more than 50,000 cases for six days in a row. Authorities have recorded more than 50,000 new infections a day since passing that milestone for the first time on December 29.
Moving ahead of Johnson, Scottish First Minister Nicola Sturgeon earlier on Monday imposed the most stringent lockdown for Scotland since last spring.
He said the aim would be to "steadily move out of lockdown" and reopen schools after the February half-term.
His remarks come as Britain began rolling out the Oxford-AstraZeneca vaccine.
The Oxford/AstraZeneca vaccine is far cheaper than its rivals, costing about £2.50 ($3.40, 2.75 euros) per dose, making it affordable for developing countries.