The UK's death toll could reach 100,000 if the lockdown is lifted too early, a leading scientist has warned.
Government advisor Prof Neil Ferguson, of Imperial College London, said that even if just young and healthy people go back to work, the risk would be huge.
If that happens, the death toll could be more than five times the current confirmed level by the end of the year, he said.
It comes as stand-in PM Dominic Raab said the country is at a "delicate and dangerous stage" of its Covid-19 response, and admitted the government had to be cautious.
If there is a spike, the UK would be forced into a lockdown, Mr Raab said this morning.
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Prof Ferguson said he had been surprised how many people had stuck to the lockdown, but said politicians must be careful about what they do next.
The country's death toll in hospitals currently stands at 20,319.
He told UnHerd : "In practical terms, you would require a very high level of effective shielding for that to be a viable strategy.
"If you just achieve 80 per cent shielding - and 80 per cent reduction in infection risk in those groups - we still project that you would get more than 100,000 deaths this years from that kind of strategy.
"The most vulnerable people are also the people who most need care and most need interaction with the health system and are are least able to be truly isolated."
He said that the lockdown appeared to be working, with infection rates falling.
In comparison, he said, the death toll and number of cases is on the rise in Sweden, which has not gone into lockdown.
Speaking on Sky News’ Ridge on Sunday, First Minister Mr Raab warned that the next steps need to be “sure footed” as another lockdown would be a “serious blow to public confidence”.
He added that the government is doing its “homework” before announcing what the next phase of the coronavirus response will be.
“We are at a delicate and dangerous stage and we need to make sure that the next steps are sure-footed,” Raab said, adding that the government was “doing the homework” behind closed doors on what would happen in the next phase.
“It’s not responsible to start speculating about the individual measures,” he said, urging the public to stick to the current guidance, which is to stay at home except for essential travel.
“We won’t just have this binary easing up of measures. We will end up moving to a new normal,” he told BBC1’s The Andrew Marr Show.
“We need to take a sure-footed step forward which protects life but also preserves our way of life.
"So we are very focused on doing the homework that can allow us to do that.”
He said it was “inconceivable” schools could re-open without measures in place to stop the spread of the disease, but said ministers were looking to ease restrictions on outdoor activities.
“We do want to look - when it is safe, when it is responsible - at ways to allow more outdoor activities to take place, but again we have got to have the evidence that that is a sure-footed step - doesn’t allow coronavirus to get a grip back on the country.”
