LONDON—On Thursday, Boris Johnson delivered the most chilling warning from a British prime minister since Winston Churchill prepared the country for potential destruction during World War Two. “I must level with you,” Johnson said, looking like a man who had just emerged from a doctor’s office after receiving a terrible diagnosis. “Many more families are going to lose loved ones before their time.”

His message was stark, and somehow seemed to hit even harder coming from a man who has built his political career on flippant clownery. The novel coronavirus can no longer be contained in the United Kingdom, he said, and all that can be done now is to mitigate the worst effects of it to give the doctors and nurses in the country’s already creaking national health service a fighting chance to save as many lives as possible.

But the strict containment measures now familiar around the planet—population lockdowns, prohibition of mass gatherings, and travel bans—were notable only by their absences. Johnson’s government has diverged with the rest of the world and decided to take a much less draconian approach, and his scientific advisers have admitted the strategy could see as much as 60 percent of the population catch the coronavirus.

In point of fact, Britain is effectively encouraging a potentially deadly virus to spread to the majority of the people who live here. Remember, the U.K. is one of the only European countries which is still allowed to send flights to the United States under President Donald Trump’s travel ban. That will change, however, come Monday night when a travel ban on the U.K. and Ireland will kick in, according to a Saturday announcement by Vice President Mike Pence.

The logic goes like this: if a large section of the population is exposed to the coronavirus now, it will likely help people develop a level of immunity, and then Britain will be in the best possible position to defend itself from a much worse outbreak of the virus in the future. The strategy reportedly has been shaped by the history of the deadly Spanish Flu epidemic of 1918, which saw a second, much larger spike in deaths months after the first outbreak.

Sir Patrick Vallance, England’s chief scientific adviser, said the government was looking “to build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission.” The strategy is based on science—but the entire British population is now effectively taking part in a mass untested experiment, and one which could result in the unnecessary deaths of hundreds of thousands of people.

Sixty percent of the British population is just under 40 million people. Even if Britain experiences a low mortality rate from the virus, that would lead to an expected 300,000 deaths. If the mortality rate is higher, such as in Italy, it’s not unthinkable that the number of deaths rises over a million.

While the strategy has its supporters in the scientific community, many have reacted with sheer horror. Anthony Costello, a pediatrician and former World Health Organization director, wrote that it’s not even clear yet that catching the coronavirus will result in immunity. Costello urged the U.K. to change course, asking: “Is it ethical to adopt a policy that threatens immediate casualties on the basis of an uncertain future benefit?”

Tedros Adhanom Ghebreyesus, the current director-general of the World Health Organization, has also urged countries to continue with containment measures. “The idea that countries should shift from containment to mitigation is wrong and dangerous,” he said. “We urge all countries to take a comprehensive approach tailored to their circumstances—with containment as the central pillar.”

Other scientists have said Johnson appears to have missed the point when it comes to containment, saying that if the spread of the disease is delayed for enough time, an effective therapy or treatment could be developed and make the virus much easier to defeat, without risking as many lives.

Johnson’s gamble also has been questioned by his political allies. Jeremy Hunt, the former health secretary who remains a high-profile lawmaker in Johnson’s own Conservative party, described the prime minister’s approach as “surprising and concerning” and warned that it could lead to Britain’s outbreak becoming worse than Italy’s within a matter of weeks.

And it’s not just British lives at risk from the strategy. Flights from Britain were, mystifyingly, still free to cross the pond to the U.S. despite Trump’s decision to ban journeys from the rest of Europe. Trump previously said he decided to exclude the U.K. because it was “doing a good job” on battling the virus, but admitted Friday that he now may have to add it to the list. On Saturday, he included the U.K. in the ban.

Following days of criticism, Johnson has indicated that he may tweak his strategy. His government has banned hundreds of local elections and the London mayoral election for a year, and British media reported Friday night that mass gatherings could be banned from next weekend.

But these tentative moves come as governments around the world declare emergencies, close borders, shut schools, impose strict entry and quarantine requirements, and carry out mass testing on their populations.

Johnson has chosen a wildly different course from governments around the world. The question now is if his mass experiment will define his legacy as an ingenious pioneer, or as the mad professor who foolishly gambled with the lives of millions.