Downing Street is privately working on the assumption that the second wave of coronavirus will be more deadly than the first, with the death toll remaining high throughout the winter.
An internal analysis of the projected course of the second wave is understood to show deaths peaking at a lower level than in the spring but remaining at that level for weeks or even months.
It is understood that the projection – provided by the Scientific Advisory Group for Emergencies (Sage) – has led to intense lobbying from Sir Patrick Vallance and other Government advisers for Boris Johnson to take more drastic action.
"It's going to be worse this time, more deaths," said one well-placed source. "That is the projection that has been put in front of the Prime Minister, and he is now being put under a lot of pressure to lock down again."
Separately, Sage has warned that all of England will need to be under Tier 3 Covid-19 restrictions by mid-December scuppering Boris Johnson's hopes for a normal Christmas.
It comes as countries across Europe battle rapidly rising case numbers and prepare to introduce new measures.
Angela Merkel, the German chancellor, is considering restaurant and bar closures and will hold talks with state leaders about additional restrictions on Wednesday.
France is looking at a weekend curfew amid warnings of a "brutal" rise in infections. The country recorded a death toll of more than 500 on Tuesday and President Emmanuel Macron will address the nation on Wednesday evening, with such broadcasts having previously been used to announce new measures to fight the virus.
Watch: Angela Merkel is reportedly considering a nationwide 'lockdown light' in Germany
Sweden, which resisted lockdown during the first wave, recorded its highest number of cases this year on Tuesday, with authorities there introducing more restrictions.
Details of the UK projection emerged as the Government announced that a further 367 people had died with Covid – the highest daily figure since May, bringing the UK total to 45,365.
Dr Yvonne Doyle, the medical director of Public Health England, said: "We continue to see the trend in deaths rising, and it is likely this will continue for some time. Each day we see more people testing positive and hospital admissions increasing.
"Being seriously ill enough from the infection to need hospital admission can sadly lead to more Covid-related deaths."
Health officials expect the death toll to reach 500 a day within weeks.
Leeds Teaching Hospitals NHS Trust, which runs Leeds General Infirmary and St James's Hospital, has cancelled all bar essential operations as services come under sustained pressure.
In an internal memo, the hospital's deputy chief medical officer, David Berridge, said a 78 per cent rise in admissions in the last week "means it is looking even more likely that Leeds will move into Tier 3 following discussions across the city and with the Government."
Doncaster and Bassetlaw hospitals have seen admissions double in a week, Health Service Journal reported, with closures of operating theatres in Mid Yorkshire Hospitals Trust.
While the first Covid wave in the UK saw a sharp spike in deaths, peaking in April, Mr Johnson has been warned that the second wave could last significantly longer. Government scientists fear deaths could remain in the hundreds for at least three months and continue long after Christmas, even if the current restrictions remain in place until March.
The Government has come under intense pressure from many Tory MPs to abandon its three-tier system of lockdowns and prioritise the economy instead.
But Mr Johnson has refused to back down from his policy and made it clear that even tougher restrictions may be needed. He has said the country is treading a "narrow path" and that a "circuit-breaker" lockdown remains on the table.
The growing concerns of Government scientists may explain the Prime Minister's determination to stick to a policy that has proved so controversial. They may also shed light on the reluctance of ministers to discuss an "exit strategy" from the current restrictions.
More than eight million people – almost all of them in the North – are now living under Tier 3 measures, with no end in sight.
Professor Wendy Barclay, a Sage member and scientist from Imperial College London, on Tuesday said none of the current restrictions appeared to be having a significant impact on the spread of the virus.
"The total lockdown that we had back in late March was enough to turn the tide and get the virus back under control," she told Times Radio. "So far, none of the other restrictions that we've seen, and none of the other actions, seem to have done that."
On Tuesday, the World Health Organisation (WHO) said Europe's daily Covid deaths had risen by almost 40 per cent in a week. The organisation's Dr Margaret Harris said France, Spain, the UK, the Netherlands and Russia had seen some of the most significant rises.
Watch: How will England's three-tier local lockdown system work?
"Across the European region, we're seeing an intense and indeed alarming increase in cases and deaths," she told the BBC's World at One programme. Daily cases rose by a third compared with the previous week, she said, while daily deaths increased by "close to 40 per cent".
"Despite better management of hospital capacity, hospitals in several countries are filling up fast," Dr Harris said, adding that hospitals were braced for a "grim" period but experience of treating the virus in the first wave should boost survival rates.
It came as British analysis of more than 21,000 hospital admissions showed that, between March and June, death rates in intensive care from Covid had halved from 41 per cent to 21 per cent.
The findings, in research led by the University of Exeter, involving the University of Warwick and supported by The Alan Turing Institute, were published in Critical Care Medicine.