The UK will go into a four-week national lockdown from Thursday to prevent the National Health Service from being overwhelmed by COVID-19 infections.
British Prime Minister Boris Johnson made the announcement at a Downing Street press conference.
Without drastic action, "For the first time in our lives the NHS might not be there for us and our families", Johnson warned, as doctors and nurses would have to make decisions about who receives the highest levels of care.
“From Thursday until the start of December you must stay at home, you may only leave home for specific reasons,” the PM spelled out.
Most businesses including non-essential shops, pubs and restaurants - except for take-aways - will close until the start of December, but schools and universities would stay open, and work on construction sites can continue. Premier League football matches will still be played, Johnson clarified.
Moreover, supermarkets will still be able to sell 'non-essential' goods, including toys and electronics in the run-up to Christmas - unlike under the controversial 17-day 'firebreak' lockdown ordered by the Welsh government.
Crucially, the government's furlough scheme to support jobs where staff cannot work due to restrictions would be extended another month until the end of November.
Government Chief Scientific Advisor Sir Patrick Vallance told the Downing Street press conference that if the current trend of hospital admissions continued, within six weeks they would exceed the peak during the first lockdown this spring.
And he said the reproduction rate - or R-rate - was well over one, meaning the number of cases would increase exponentially.
"The key is getting the R really below one in these four weeks," Vallance stressed. "If we all do what we're supposed to do during this, the R will come down."
But he warned that some restrictions would have to continue at least until the spring.
"If we did not act now, then the chances of the NHS being in extraordinary trouble in December would be very high," Chief Medical Advisor Chris Whitty added.
But he expressed optimism for a fall in cases in the spring after the flu season, and for pharmaceutical advances. "There are now multiple shots on goal from science," he said, including work towards a COVID-19 vaccine.
"No responsible prime minister could ignore the message of those figures," Johnson said.
"No-one wants to be imposing these kinds of measures anywhere," he added, noting the effects on businesses and on individual mental health. But he stressed that weekly death tolls could hit 7,000, and even in the South-West, which has been largely spared during the pandemic, hospitals could run out of capacity in weeks.
Earlier this week the makeshift 'Nightingale Hospital' in Manchester - one of nine such facilities nationwide - was reopened to accept non-COVID patients.
Johnson said Parliament would vote on the measures on Wednesday, in line with earlier assurances following a rebuke to the prime minister from House of Commons Speaker Sir Lindsay Hoyle.
Shadow Public Health Minister Alex Norris said Labour would be "inclined" to support the measures in Wednesday's vote.
The PM originally planned to unveil new measures in response to a spike in cases on Monday but was forced to bring the press conference forward after the plans were leaked to the media on Friday.
The change in policy from the current three-tier system of region-by-region and city-by-city graded restrictions was prompted by fears of the National Health Service (NHS) being overwhelmed by a combination of a new coronavirus surge and the annual winter flu season.
The Conservative government had resisted calls for a total lockdown from some public health experts and the opposition Labour Party's demand for a two-week 'circuit-breaker' lockdown.
The lockdown would override the current three-tier system, but that could be reinstated to bring some areas out of lockdown sooner than others.
Earlier on Saturday Tory backbench MP Steve Baker, a leading Brexiteer and lockdown-sceptic, visited Johnson at 10 Downing Street along with "three scientists and a data analyst" on Saturday afternoon for a briefing. Asked by reporters later if there would be a new lockdown, he replied: "The truth is there's more nuance than that here."
"The prime minister's got very, very difficult choices to make, and I would encourage all members of the public and MPs to listen extremely carefully to what the prime minister says today and over coming days," Baker added.
"And unlike the first wave, where we had a national lockdown which protected huge swathes of society, this outbreak is now running riot across all age groups."
But former Times editor Andrew Neil pointed out that infection rates were only rising for over-45s.
Sky News interviewed a tearful shopkeeper in Great Torrington, in the north of Devon, where there were no more than five coronavirus cases in the whole town. "Not the whole country. They should've kept people in their own units and their own counties. It's just bad," she sobbed, expressing frustration that small high-street businesses would have to close while out-of-town superstores served hundreds of customers at once.