In an “emergency” press conference in which some Covid-19 experts spoke out against the government allowing mass infection across the population, the prime minister was told: “Those who claim victory too easily will fail.”
It comes after Johnson announced on Monday that most mitigations against the virus will be dropped when the lockdown ends.
This end date is currently set for 19 July, with final confirmation from the PM due on Monday.
On Thursday, medical journal The Lancet published a piece, signed by 97 scientists and experts, warning lifting restrictions will be “dangerous and premature”.
The government itself has acknowledged Covid cases will soon reach 100,000 a day as restrictions are lifted.
In the press conference arranged following the publication of The Lancet article, editor Dr Richard Horton pointed to modelling from the Institute for Health Metrics and Evaluation suggesting nearly 10,000 people could die from the disease by 1 October.
The same institute published modelling last month saying nearly 50,000 people could die of Covid by this date. It's not clear if Dr Horton was in fact referring to this research.
Different modelling presented to the government’s Scientific Advisory Group on Emergencies (Sage) has suggested the summer death toll could be 40,000.
“In other words,” Dr Horton said, “we are at a very dangerous moment in this pandemic. So let's not be epidemiologically stupid."
He said vaccine coverage needs to be scaled up to 70% of the total population – it’s currently 50% – and social distancing and mask wearing rules should continue.
“If we don’t do this, if we continue with the plan to lift mandates on 19 July, it will not be freedom that we win, it will be a self-inflicted wound of uncontrolled transmission of the Delta variant.
“The overriding lesson from countries worldwide is that those who claim victory too easily will fail. And the government’s plans currently fail all that we know about the science of coronavirus.”
Dr Deepti Gurdasani, a clinical epidemiologist from Queen Mary University of London who convened the press conference, accused the government of “effectively” pursuing a herd immunity strategy by “letting the pandemic rip through the unvaccinated”.
Dr Gurdasani said lifting restrictions will “create a generation blighted by a disease… they could have been vaccinated against in just a matter of weeks”.
“This government’s strategy is callous and inhumane. The government has completely abdicated its responsibility to the public.”
Johnson, for his part, has regularly pointed to the vaccination programme preventing huge numbers of Covid hospital admissions and deaths.
While these numbers are increasing, they are still nowhere near the levels seen during the second wave of the pandemic – though on Wednesday, daily cases were still 67,000 below the number anticipated by the government later this summer.
In an apparent challenge to his critics on Monday, Johnson asked: “If we can't reopen our society in the next few weeks... we must ask ourselves when will we be able to return to normal?"
England’s chief medical officer Prof Chris Whitty, the UK's most prominent Covid scientist, also said he has “quite a strong view” lifting lockdown in the summer has advantages over autumn.
He said autumn will be “when schools are going back and when we’re heading into the winter period when the NHS tends to be under greatest pressure for many other reasons”.
Prof Neil Ferguson, the scientist whose modelling convinced the PM to impose the first lockdown in March last year, also said this week that lifting lockdown, while a "gamble", is "justifiable".
He said he is "reasonably optimistic, but policy will have to remain flexible".