Only shielding the elderly and vulnerable from coronavirus could see 100,000 deaths this year, a leading expert has warned.
Professor Neil Ferguson, from Imperial College London, says he is skeptical that a scenario where the younger population resumed a normal life would be a "viable strategy".
It comes as the UK's Prime Minister Boris Johnson returned to work on Monday and warned the lockdown would not be lifted anytime soon.
The UK's month-long lockdown has been extended until May 7 and the government is due to announcement its plans for easing it in the coming days.
Prof Ferguson has been one of the leading scientists advising the government.
He has said easing the lockdown would require "a very high level" of effective protection for the vulnerable and elderly population, who are also the "least able to really be truly isolated".
"The most vulnerable people are also the people who most need care and have most interaction with the health system and are least able to really be truly isolated," he told news outlet UnHerd.
"And if you achieve just 80 per cent shielding, 80 per cent reduction in infection risk in those groups, we'd still project you get well over 100,000 deaths later this year through that sort of strategy."
Earlier, Prof Ferguson had warned the pandemic could kill 250,000 people if the government did not enforce social distancing.
He has advised that everyday life "cannot go completely back to normal" and that social distancing would need to remain until a vaccine is developed.
Last Thursday the UK began clinical trials on testing vaccines but it is not expected one will be available until next year.
"There will be a trade-off between the extent to which you relax measures and tolerate a level of transmission, and, therefore, mortality and health system demand, versus keeping case numbers as low as you can, which will probably require social distancing longer term," Prof Ferguson added.
On Monday, Mr Johnson warned that if measures were eased too early and gave way to a second wave it would be an "economic disaster".
"I understand your impatience, I share your anxiety and I know that without our private sector without the drive and commitment of the wealth creators of this country there will be no economy to speak of," he told the nation.
"There will be no cash to pay for our public services, no way of funding our NHS and yes I can see the long term consequences of lock down as clearly as anyone and yet we must also recognise the risk of a second spike, the risk of losing control of that virus because that would mean not only a new wave of death and disease but also an economic disaster.
"We would be forced once again to slam on the brakes across the whole country and the whole economy and reimpose restrictions in such a way as to do more and lasting damage and so I know it is tough and I want to get this economy moving as fast as I can but I refuse to throw away all the effort and the sacrifice of the British people and to risk a second major outbreak and huge loss of life and the overwhelming of the NHS."
Mr Johnson said a decision on the lifting of the lockdown will depend on scientific advice.
"We simply cannot spell out now how fast or slow or even when those changes will be made," he added
"Though clearly the government will be saying much more about this in the coming days."
It is exactly one month since Mr Johnson went into isolation after contracting a severe case of Covid-19, which put him in intensive care and left the UK without its leader as the country entered the peak of the outbreak.
