People face up to six months of coronavirus restrictions after a huge surge in cases.
Government experts are planning for the new ban on gatherings larger than six people to be in place for at least three months, the Mirror understands.
And if there is no change in circumstances, such as a vaccine breakthrough, they fear restrictions could be in place for six months.
The move spells a grim winter ahead – and puts Christmas under threat for millions.
It came as Boris Johnson confirmed it will be illegal to gather in groups of more than six in England from Monday.
In a No10 press conference, the Prime Minister finally admitted he was forced to “simplify” the rules after they “have become quite complicated and confusing”.
Chief Medical Officer Chris Whitty today declined to say exactly how long the new ban on gatherings will last.
But he said “it’s very unlikely to be over in just two or three weeks.”
Mr Johnson claimed: “I’m still hopeful that in many ways we could be able to get some aspects of our lives back to normal by Christmas.”
But Prof Whitty told a Downing Street press conference: “The period between now and spring is going to be difficult because this is a respiratory virus.”
Mr Johnson admitted the hard limit of six people will hit large families who can no longer have all the grandparents round for Sunday lunch.
The PM said it “breaks my heart”.
He added: “I’m sorry about that and I wish we did not have to take this step. But as your Prime Minister I must do what is necessary to stop the spread of the virus and to save lives.”
But he told a Downing Street press conference “we must act” – and claimed the new restrictions could help avoid a second full national lockdown.
There will be exemptions for weddings and funerals, work and school, and for households or “support bubbles” that are larger than six people.
It comes after England’s cases surged from 12.5 per 100,000 people to 19.7 in a week.
The incidence rate is far higher for the young – at 48.1 for those aged 17 and 18, 54.5 for 19 to 21 year olds, and 41.6 for those aged 20 to 29.
The government’s scientists now believe the ‘R’ – the number of people infected by each Covid-19 carrier – is above 1, but haven’t announced how much by yet.
A senior government source warned: “This is the time to act. We’re not yet in trouble but we’re definitely heading for trouble when it comes to the potential for this having an impact on the NHS.”
Earlier, Health Secretary Matt Hancock admitted the new law may stretch “potentially beyond” Christmas.
Mr Johnson did not put a time limit on the ban on large gatherings, saying he would keep it “under constant review and only keep it in place as long as is necessary.”
However, it’s understood officials are planning for it to be in place for at least three months.
Officials fear the drawing in of the seasons will ramp up the potency of the virus because more people will crowd into poorly-ventilated spaces.
Chief Scientific Advisor Sir Patrick Vallance told a No10 press conference it was “inevitable” that there will be more infections in winter.
And a senior government source said: “If you look over the next three months, there is nothing that is going to work in our favour and several things that may to work against us.”
They added: “We would expect there to be a difficult period over the next six months unless we get major breakthroughs like a new vaccine.”
The huge rise in virus in young people comes before the vast majority of England’s universities return to in-person teaching in the coming weeks.
This trend is also repeated in the proportion of people in these age groups testing postive for the virus, out of those who get a test.
Some 6.1% of 17 and 18-year-olds who went for a test recently saw it come back positive.
The figure is 5.1% for those aged 19 to 21, and 3.7% for those aged 20 to 29.
Nationally the average percentage of tests coming back positive in England has gone up to 6%, up from 2.5% a couple of weeks ago.
Prof Whitty told the No10 press conference: “The graphs are going up – you can’t argue with those numbers”.
Currently, the only group not seeing an increase is children under 17 – leading ministers to insist schools are safe.
In other developments today, England’s pubs and restaurants will be forced to take punters’ contact details or face £1,000 fines in a coronavirus crackdown.
Hospitality venues, which also include cafes, have been told since July to keep customers’ details for 21 days for contact tracers.
But until now the policy was only voluntary in England – despite being made a legal requirement in Scotland.
Health Secretary Matt Hancock confirmed it will now be a requirement in England too.
Now it is understood those who do not comply will face a £1,000 fine.
Meanwhile the return of fans attending live sport has been hurled into doubt after coronavirus cases soared in England.
Boris Johnson today announced he will “review” plans to return audiences to stadiums from October 1.
Meanwhile, the Prime Minister will also “revise” plans to pilot larger audiences in arts venues like theatres later this month.
Mr Johnson told a No10 press conference: “That doesn’t mean we’re going to scrap the programme entirely.
“It just means we are going to review and abridge it, and the Culture Secretary will say more shortly.”
It is another devastating blow to clubs who hoped a return of fans in the winter could help their budgets.
It’s understood some pilots of live audiences for arts or sport will still go ahead.
However, they will be limited to 1,000 people and will not go ahead at all if they’re in areas which had high virus rates.
At PMQs, Mr Johnson repeatedly blamed the public for overloading the Covid-19 testing system by ordering tests when they don’t have symptoms.
He said: “I, of course, sympathise with all those who are facing difficulties getting a test as fast as they want but demand is at an unprecedented high, particularly because of demand for asymptomatic patients.”
Health Secretary Matt Hancock also blamed the British public for getting too many coronavirus tests – after the number of daily positive Covid-19 cases in the UK rose to almost 3,000.
He attempted to shift the blame after people with coughs and fevers were told to travel more than 300 miles to get tested for Covid-19.