Allowing the young to go back to normal while shielding the elderly would have 'dire consequences' for the NHS, SAGE has warned.
In leaked documents - handed to ministers last week - the advisory panel suggested allowing them to pack out bars and night clubs was 'not viable' because it would trigger an enormous burden of hospitalisations, deaths and knock-on non-Covid-19 fatalities in all age groups.
They said it would not be possible to prevent contact between age groups, with even 'small leaks' in infections leading to 'many illnesses'.
The top epidemiologists also dismissed calls for the UK to follow a herd immunity strategy, warning an 'unconstrained epidemic' in the young would seriously impact health services.
They pointed to the fact a quarter of all hospitalisations in the first wave were under 60, and that 'shielding strategies' in care homes had still resulted in more than 20,000 deaths, to back up their arguments.
But scientists remained divided on the issue, with almost 11,000 medical and public health academic backing the Barrington Declaration that calls on ministers to dodge lockdown restrictions and follow a 'herd immunity' strategy.
The declaration says it would be better to 'allow those who are at minimal risk of death to live their lives normally to build up immunity to the virus through natural infection, while better protecting those who are at highest risk'.
It comes after a study by King's College London found one in ten people under 50 could be struck by 'long Covid' - suffering chest pains, breathing problems and chronic fatigue - if they contract the virus.
SAGE has warned a strategy of an 'unconstrained epidemic' in an age group will not work because there will be 'leaks' in infections to others. Pictured: Revellers in Manchester enjoy a night out ahead of tighter lockdown restrictions
It comes after an 83-year-old shopper from Barnsley, South Yorkshire, said she 'couldn't give a sod' about new restrictions coming into force in the area on Saturday
An 83-year-old woman from Barnsley has said she doesn't 'give a sod' about staying at home after it was announced Tier 3 restrictions would come into force this weekend.
The outspoken shopper told the BBC =: 'I think it's all ridiculous, we should never have been in lockdown. All the people who were vulnerable should have been helped and kept home safe.
'And all the rest of us, i'm 83, I don't give a sod.
'I look at it this way, i've not got all that many years left of me and i'm not going to be fastened in a house when the government have got it all wrong.
'We need...how can we get the country on its feet? Money-wise? Where's all the money?
'By the end of this year there's going to be millions of people unemployed and you know who's going to pay for it? All the young ones. Not me because i'm going to be dead.'
From midnight Saturday the South Yorkshire areas of Barnsley, Doncaster, Rotherham and Sheffield will move under Tier 3, joining Manchester, Lancashire and Liverpool.
In the documents - dated October 15 and seen by The Times - SAGE says: 'Such a strategy (of segmenting age groups) would not be viable because it would not be possible to prevent the virus spreading from younger people to older people.
'A very large proportion of the population need to withdraw from daily life for many months, which would have profound consequences for the NHS as well as having unknown long-term effects in those infected.'
It added: 'We do not know if long-term immunity results from infection with SARS-CoV-2; even if high levels of immunity could be achieved within the younger age group, it is almost certain that a further epidemic wave in older people would happen once segmentation ended.'
The top scientists also dismissed the possibility of segmenting the population in July, according to minutes of meetings published by the Government last week.
They said any move to split the populations by age is 'likely to fail', after modelling revealed a high level of contact in younger groups between those over 45.
But it also showed that people over 70 tended to mix with their own age group far more than with any other.
'Policies to segment the population by age, relaxing restrictions for younger groups while restricting them for older groups, are likely to fail,' they advised the Government.
'The large amount of mixing across age groups would make it extremely difficult to prevent transmission between segmented age groups, regardless of the ethical and practical questions involved.
'Further, any age segmentation policy will surely raise considerable social, ethical and practical issues not considered here, in addition to costs to wider well-being.'
Minutes from July published last month also advised against plans for everyone over the age of 45 to shield, saying around two-thirds of people in the UK live in a household including at least one person in this age bracket.