LONDON — Britain's neighbors in Europe are shutting down the continent to confront the spread of the novel coronavirus: locking pubs in Dublin and cafes in Paris, closing schools, enacting curfews and enforcing quarantines not seen since the Middle Ages.

But in London, the bars are still open. Most schools, museums and restaurants are, too.

Unlike Italy, France and Spain, the British government hasn’t stopped anyone from going anywhere. But that go-slow approach by Britain began to shift on Monday when Prime Minister Johnson encouraged his fellow citizens to avoid “all non-essential contact with others,” to work from home and to self-isolate if they are elderly. All the measures are voluntary, but Johnson warned that his government had the power to make them mandatory.

For days, as the continent began to shut down, Johnson’s government made cautious moves to try to slow the virus, charting its own course, resisting the tough restrictions adopted by its neighbors.

The government acknowledges it will have to take more draconian steps eventually — but it doesn’t want to act too soon.

“We’re in a long game,” said Chief Medical Officer Chris Whitty at a Monday news conference.

Johnson and his team are betting that the pandemic lasts for many months, or longer. And they have worried that asking citizens to stay at home and avoid social contact is “very difficult to maintain over a long time,” Whitty said.

The timing is everything, said Chief Science Officer Patrick Vallance, who has expressed concern about “behavioral fatigue.”

“If you tell people to stay at home too early,” Vallance has said, “they get fed up with this at the very point where you need them to stay at home.”

But even as they defend their past strategy, Johnson and his advisers face the dual pressures of a surging coronavirus caseload and a growing chorus of critics, in politics and science, who want more done now. As of Monday, Britain had more than 1,550 confirmed cases of coronavirus and had reported 35 deaths.

“We are losing time,” warned Devi Sridhar, chair of global public health at the University of Edinburgh’s Medical School.

She called the British government’s actions up until today “worrying and possibly reckless” in an opinion piece, saying “while it might work in an ideal, predictable world, or in a computer simulation, we don’t live in a computer simulation.”

Some 700 British psychologists signed a letter stating that fear of “behavioural fatigue” was not a good enough reason to go slow on social distancing mandates.

Urike Hahn, a behavioral psychologist at the University of London, said it was an open question whether Britons would tire or rebel against government prohibitions, especially those designed to protect them against an infectious disease with a mortality rate estimated as high as 3.4 percent.

“People will only know they are living in an exceptional situation when they see exception measures taken,” she said.

Johnson said his government — like every other government now — is seeking to delay the spike in new cases, to “flatten the curve,” or as the prime minister put it when pointing to a chart, “to squash the sombrero.”

Britain’s single-payer, government-supported National Health Service is beloved by many here, but its doctors warn that a large wave of cases, sustained over weeks or months, could overwhelm an already stressed system.

Even before the new virus struck, there were few empty beds at NHS hospitals, as the seasonal flu was still in swing.

More worrying, the NHS has only 5,000 ventilators on hand for a population of 66 million, one of the lowest numbers per capita in Europe. The life-giving oxygen machines are so scarce in Britain that the health secretary revealed Sunday that the government wants auto manufacturers and army suppliers to convert their assembly lines to making ventilators.

Spain announced it was moving to lock down its borders, telling residents to mostly stay home on March 14 in hopes of slowing the spread of coronavirus. (Raul Gallego Abellan/The Washington Post)

For Johnson and his scientific advisers, the most controversial part of their evolving plan is the impression, poorly communicated, that Britain was resigned to seeing massive numbers of infections until the population has built up enough antibodies to produce “herd immunity.”

The NHS is focusing its testing on those checking into hospitals. Public health workers have mostly stopped tracing contacts of the infected.

Many people were stunned to hear Chancellor Angela Merkel say last week that in Germany 60 to 70 percent of the population could eventually be infected by the novel coronavirus. Vallance said Merkel was being too conservative. The United Kingdom could see 80 percent of its citizens could be infected, he said, in “a reasonable worst-case scenario.”

Vallance earlier told the BBC, “Our aim is to try to reduce the peak, broaden the peak, not suppress it completely; also, because the vast majority of people get a mild illness, to build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission, at the same time we protect those who are most vulnerable to it.”

He added: “If you suppress something very, very hard, when you release those measures it bounces back and it bounces back at the wrong time.”

Many took this to mean the government accepted that a very large number of their citizens will be infected with the novel coronavirus and that once the majority of them recover from their mild to moderate symptoms, they will be have enough antibodies coursing in their bodies to produce what infectious-disease specialists call “herd immunity.”

The people cried out that they were not a herd. And new modeling by epidemiologists stressed that as many as 250,000 could die unless the government devoted itself to suppresing the epidemic.

In an open letter of protest, signed by more than 500 British physicians and scientists, including prominent infectious-disease specialists, the experts warned that the growth curves for infection in Britain were in line with Italy, Spain, France and Germany — and that numbers in this initial phase are doubling quickly.

They argued that Britain should immediately enforce social distancing measures, so “thousands of lives can be spared.”

“Going for ‘herd immunity’ at this point does not seem a viable option, as this will put NHS at an even stronger level of stress, risking many more lives than necessary,” the group said, warning that the current voluntary social distancing efforts were “insufficient” and that “more restrictive measures should be taken immediately, as is already happening in other countries across the world.”

Adam Kucharski, a mathematician and epidemiologist at the London School of Hygiene and Tropical Medicine, whose group is advising the government, wrote that he was “deeply uncomfortable” with the message that Britain is actively pursuing herd immunity as a main strategy. “Herd immunity has never been the outright aim, it’s been a tragic consequence of having a virus that — based on current evidence — is unlikely to be fully controllable in long term,” he wrote in a tweet. “Talk of ‘herd immunity as the aim’ is totally wide of the mark.”

Criticism of Britain’s approach comes not only from home but abroad. World Health Organization officials say testing and tracing should continue, that strict social distancing should be enforced and that awaiting herd immunity is not a proven remedy.

On Monday, Chinese state media criticized Britain and other Western nations for not enacting more rigorous controls., saying they have adopted a policy of “total surrender.”