A strict lockdown for the over-60s was considered by Government scientists to allow Covid immunity to build up in the younger population but was dismissed as unworkable, a Sage paper reveals.
Sir Patrick Vallance, the Government's chief scientific adviser, asked experts to put together an advice document about the feasibility of letting the virus run rampant while shielding older people.
Such a plan has been backed by thousands of clinicians and scientists worldwide who have signed the Great Barrington declaration, which asks governments to allow herd immunity to build while protecting the most vulnerable.
However, Sage concluded that the plan was unfeasible and warned that locking away the over-60s for many months would have a negative impact on their lives as well as "substantial" legal and ethical ramifications.
The document, published on October 15, considered "adding further restrictions to those over 60 or who are otherwise identified as vulnerable, while not attempting to restrain transmission in younger people".
But it warned that it would be impossible to prevent the virus spreading from younger people to older even with extra shielding measures in place and said asking a very large proportion of the population to "withdraw from daily life" would have a "profound negative effect".
Experts said an uncontrolled epidemic in younger age groups would have "dire consequences" for the NHS and could leave those infected with long-term effects.
"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 occur once segmentation ended," the scientists concluded.
The Government's original strategy was to keep infections to a lowish level while allowing some herd immunity to build up in the population and avoid a deadly second wave. But after modelling showed that the NHS would be overwhelmed without immediate suppression, the country was placed into lockdown in March.
Commenting on the document, released among a new tranche of Sage papers on Friday, Professor James Naismith, director of the Rosalind Franklin Institute at the University of Oxford, said proposals to segment large parts of society would not work.
"Millions of people, including key workers, would have to isolate. The consequences for them and the rest of us would be severe,"he said. "Since multi-generational households would have to be isolated, this would disproportionately burden the non-white population."
Dr Michael Head, senior research fellow in global health at the University of Southampton, said: "The idea is to reach a level of herd immunity in the population by allowing the spread of Covid-19 through much of society whilst trying to protect vulnerable populations. In my opinion, it is a very bad idea.
"It is good to see a consensus statement from Sage reiterating that this segmentation idea is a strategy that has no place in our society. We have seen during a full national lockdown that vulnerable people could not be easily shielded, with a great deal of excess death.
"There is also the emerging evidence around long Covid, and issues of waning immunity and reinfection."