Britain’s initial response to the Covid-19 pandemic “ranks as one of the most important public health failures the United Kingdom has ever experienced,” a parliamentary inquiry reported on Tuesday, blaming the government for “many thousands of deaths which could have been avoided.”

In a highly critical, 151-page report, two committees of lawmakers wrote that the government’s failure to carry out widespread testing or swiftly impose lockdowns and other restrictions amounted to a pursuit of “herd immunity by infection” — accepting that many people would get the coronavirus and that the only option was to try to manage its spread.

“It is now clear that this was the wrong policy, and that it led to a higher initial death toll than would have resulted from a more emphatic early policy,” the report concluded.

Although many of its findings were already known, the report grew out of the first authoritative investigation of Britain’s pandemic response. The inquiry, led by lawmakers from Prime Minister Boris Johnson’s own Conservative Party, described a litany of failures by his government in the months after the first coronavirus cases were detected in Britain in January 2020.

Lining up at a mass testing site in Liverpool in November 2020.Mary Turner for The New York Times
Britain has experienced one of the worst Covid-19 outbreaks among wealthy nations, with 162,000 deaths officially attributed to the disease. Like many Western democracies, at the outset of the pandemic it struggled to balance individual liberties with strict measures such as lockdowns, and suffered from mismanagement at the top levels of government.

The country has tried to put those missteps behind it, racing ahead last winter and spring as one of the world leaders in vaccinations, with more than three-quarters of people 12 and older having now received two doses of a Covid vaccine. As deaths declined from prior peaks, Britain cast off nearly all restrictions, and even though infections remain high, Mr. Johnson has tried to portray the country as having put the worst of the pandemic behind it.

But as he struggles against a raft of new economic problems, the report renewed criticisms of his government’s handling of the virus. It does not require the government to act, but its findings are likely to influence the public debate for months to come. A full public inquiry promised by Mr. Johnson is not scheduled to begin until next year.

“This report lays bare the failings of the U.K. government to contain Covid, including delayed border measures, nonexistent testing for weeks, lack of P.P.E. for frontline workers and a late lockdown,” said Devi Sridhar, the head of the global public health program at the University of Edinburgh. “Hopefully, lessons will be learned from this.”

The inquiry drew on numerous interviews over the past year with government officials and experts, many of which were held in open sessions. It concluded that while Britain had a robust system in place to detect and respond to major public health threats, it was too heavily geared toward the risk of pandemic flu and not a faster-spreading, more dangerous respiratory disease such as Covid-19, SARS or MERS.

Asian countries with more recent experience of such diseases quickly put in place aggressive containment, testing and tracing strategies. But in Britain, the government’s scientific advisers advocated a far more gradual approach, mistakenly believing “that a new, unknown and rampant virus could be regulated in such a precise way,” the report said.

The lawmakers found that such decisions were the product of “groupthink” among top officials in Mr. Johnson’s government and its advisers, who relied on mathematical models to guide their response.

On Tuesday, the government defended its actions, saying they were guided by science, and reiterated its regret for the country’s suffering. “One can’t apply hindsight to the challenges that we faced,” Steve Barclay, a cabinet minister, told BBC Radio.

But in fact, early in the outbreak, infectious disease and public health experts pleaded with the British government to take stronger measures. It was only in late March 2020, with infections doubling every three days and the national health system at risk of being overwhelmed, that Mr. Johnson reversed course and ordered a full nationwide lockdown — a week to two weeks after France, Spain and Italy had done so.

“The loss of that time was to prove fatal to many,” the report said.

Mr. Johnson, who was hospitalized with Covid in April of 2020, has consistently faced opposition to pandemic restrictions from a large faction within his party.

The inquiry found that the government’s decision not to order a lockdown or conduct extensive contact tracing was due in part to officials’ belief that the British public would not accept such restrictions — a view that was based on limited evidence and turned out to be false, as people generally complied with lockdown measures, the report said.

The assumption of noncompliance was “one of the critical things that was completely wrong in the whole official thinking,” Dominic Cummings, a former chief adviser to Mr. Johnson, told the lawmakers in testimony this spring during which he laid into his former boss for incompetence.

The decision to abandon widespread testing early in the pandemic also cost lives, especially as older people were discharged from hospitals to care homes without knowing whether they or their caregivers were infected with the virus. The low levels of testing meant that the country “lost visibility of where the disease was spreading,” the report said. The care facilities, like many hospitals, lacked protective equipment, allowing the virus to run rampant among the country’s most vulnerable people.

Peter English, a retired consultant in communicable disease control, said the government’s panel of health experts, the Scientific Advisory Group for Emergencies, was filled with “the wrong experts,” people who lacked experience with infectious diseases.

“They had lots of modelists in place, but very few people who were used to dealing with these things and managing outbreaks in practice,” he said.

The inquiry did praise certain aspects of Britain’s handling of the pandemic, in particular its early investment in vaccine research, including support for the Oxford-AstraZeneca vaccine that has become among the most widely administered in the world.

The government’s decision to lengthen the interval between vaccine doses, to enable as many people as possible to get one shot, was “decisive and courageous,” and “significantly enhanced the pace of protection for the U.K. population,” the report said.

A fuller accounting of Britain’s response is not likely to occur for years. Public inquiries like the one promised by Mr. Johnson tend to be lengthy and exhaustive. An investigation into the deadly June 2017 fire at Grenfell Tower in London, for example, has yet to conclude.

Jonathan Ashworth, a lawmaker with the opposition Labour Party, said the Covid inquiry pointed to “monumental errors” by the government.

“At every step ministers ignored warnings, responded with complacency and were too slow to act,” he said in a statement. “We need a public inquiry now so mistakes of such tragic magnitude never repeated again.”