The two Chinese companies were offering a risky proposition: two million home test kits said to detect antibodies for the coronavirus for at least $20 million, take it or leave it.

The asking price was high, the technology was unproven and the money had to be paid upfront. And the buyer would be required to pick up the crate loads of test kits from a facility in China.

Yet British officials took the deal, according to a senior civil servant involved, then confidently promised tests would be available at pharmacies in as little as two weeks. “As simple as a pregnancy test,” gushed Prime Minister Boris Johnson. “It has the potential to be a total game changer.”

There was one problem, however. The tests did not work.

Found to be insufficiently accurate by a laboratory at Oxford University, half a million of the tests are now gathering dust in storage.

Another 1.5 million bought at a similar price from other sources have also gone unused. The fiasco has left embarrassed British officials scrambling to get back at least some of the money.

“They might perhaps have slightly jumped the gun,” said Prof. Peter Openshaw of Imperial College London, a member of the government’s New and Emerging Respiratory Virus Threats Advisory Group. “There is a huge pressure on politicians to come out and say things that are positive.”

A spokesperson from the Department of Health and Social Care said that the government had ordered the smallest number of tests allowed by the sellers and that it would try to recover the money, without specifying how.

The ill-starred purchases are in some ways a parable of the risks in the escalating scrum among competing governments racing for an edge in the fight against the pandemic.

The still-emerging tests for antibodies formed in response to the virus are the next stage in the battle. By enabling public health officials to assess where the disease has spread and who might have some immunity, widespread use of the tests is seen as a critical step in determining how and when to lift the lockdownscurrently paralyzing societies and economies in much of the world.

“You can’t lift the lockdown as long as you are not testing massively,” said Nicolas Locker, a professor of virology at the University of Surrey. “As long as the government is not testing in the community, we are going to be on lockdown.”

The gamble on the Chinese antibody tests, though, is also a barometer of the desperation British officials felt as public pressure has mounted over their slow response to the virus. One prominent expert, Jeremy Farrar, the head of the Wellcome Trust, a British nonprofit that is a major funder of medical research, recently warned that “the U.K. is likely to be certainly one of the worst, if not the worst affected, country in Europe.”

Long before the development of an antibody test, Germany, for example, the continent’s leader in containing the virus, began conducting as many as 50,000 diagnostic tests a day to help trace and isolate cases. That rate is now nearly 120,000 a day.

As of Wednesday, Britain was still conducting less than 20,000diagnostic tests a day. Having missed a previous target of 25,000 diagnostic tests a day by the middle of April, officials are now promising to reach 100,000 a day by the end of the month and as many as 250,000 a day soon after that.

British officials have said that they started out behind because they lack major private testing companies of the sort found in Germany and the United States, which are capable of manufacturing and performing tens of thousands of diagnostic tests.

But by the time Britain began pushing in earnest to expand its capacity, it was also trailing behind most of Europe in the competition to buy up the limited supply of compounds, tubes and even swabs needed for diagnostic tests to determine a current infection with the virus.

So when the Chinese offers of antibody tests arrived, the officials knew that almost every government in the world was hunting for them, too. Nationalists like President Trump were pressuring domestic suppliers not to sell outside their borders. Oil-rich Persian Gulf princes were bidding up prices.

Medical companies in China, where the virus first emerged, seemed to hold all the cards, typically demanding yes-or-no decisions from buyers with full payment upfront in as little as 24 hours.

The two Chinese companies offering the antibody tests, AllTest Biotech and Wondfo Biotech, both said their products met the health, safety and environmental standards set by the European Union. Public health officials reviewed the specifications on paper while the British Foreign Ministry hurriedly dispatched diplomats in China to ensure the companies existed and to examine their products.

Representatives of both AllTest and Wondfo declined to discuss prices.

Within days of the deal, enthusiastic health officials back in London were promising that the new tests would vault Britain into the vanguard of international efforts to combat the virus.

Appearing on March 25 before a parliamentary committee, Sharon Peacock, a professor of public health and microbiology at Cambridge University who is the senior public health official overseeing infectious diseases, testified that the tests would require only a pin prick in the privacy of one’s home and would soon be available at minimal cost from either local pharmacies or Amazon.

“Testing the test is a small matter,” Prof. Peacock assured lawmakers. “I anticipate that it would be done by the end of this week.”

After quietly admitting last week that the testing had in fact proven unsuccessful, health officials are now defending the purchase as prudent planning and valuable experience.

It was to be expected, Prof. Chris Whitty, Britain’s chief medical officer, said in a news conference. “It would be very surprising if first out of the gate we got to the best outcome that we could for this kind of test,” he said. “It made a lot of sense to get started early.”

But Greg Clark, the chairman of a parliamentary committee examining the coronavirus response, said the government’s promises appeared unrealistic.

“There is no country in the world that is able to operate in massive scale antibody tests yet,” he said in an interview.

“I think it’s now clear,” he added, “that we should have moved earlier and more expansively to make use of all of the testing facilities that we could have.”

After British complaints about the test kits surfaced, both Chinese companies blamed British officials and politicians for misunderstanding or exaggerating the utility of the tests. Wondfo told Global Times, a Chinese newspaper, that its product was intended only as a supplement for patients who had already tested positive for the virus.

AllTest said in a statement on its website that the tests were “only used by professionals,” not by patients at home.

Doctors say the government’s descriptions of the antibody tests could also be misleading.

By comparing the antibody tests to pregnancy tests, officials seemed to be suggesting the antibody tests would determine whether a patient was currently infected. But a discernible level of antibodies may not appear in the blood until as long as 20 days after infection — meaning a person with the virus would test negative until then.

The British military laboratory at Porton Down is also working on an antibody test, but primarily to help public health officials assess the course of the pandemic by surveying samples of the population, not to inform individual patients. The government is hoping to repurpose some of the stored Chinese-made kits for this sort of population-level testing.

Do-it-yourself pinprick tests like the ones the British government ordered from China are far more complicated and much further off than such laboratory tests, researchers say. It is not yet certain what degree of immunity recovery from a past infection may confer, either.

Rapid antibody tests “have limited utility” for patients, the World Health Organization warned in an April 8 statement, telling doctors that such tests remained unfit for clinical purposes until they were proved to be accurate and effective.

British officials, though, were eager for a breakthrough.

The British military laboratory at Porton Down is also working on an antibody test, but primarily to help public health officials assess the course of the pandemic by surveying samples of the population, not to inform individual patients. The government is hoping to repurpose some of the stored Chinese-made kits for this sort of population-level testing.

Do-it-yourself pinprick tests like the ones the British government ordered from China are far more complicated and much further off than such laboratory tests, researchers say. It is not yet certain what degree of immunity recovery from a past infection may confer, either.

Rapid antibody tests “have limited utility” for patients, the World Health Organization warned in an April 8 statement, telling doctors that such tests remained unfit for clinical purposes until they were proved to be accurate and effective.

British officials, though, were eager for a breakthrough.

Even in late March, as the pandemic overwhelmed hospitals in Italy and Iran, British officials brushed off the advice of the World Health Organization to expand diagnostic testing as quickly as possible.

By the time Britain began pushing in earnest to expand its testing, every country in the world was competing for the same materials.

To make up the shortfall, academic research laboratories have sought to convert themselves into small-scale clinical testing facilities, typically focusing on the needs of local hospitals.

“If it comes around from the government, all well and good,” said Ravindra Gupta, professor of clinical microbiology at Cambridge University’s Department of Medicine, “but we have to prepare for nothing to come. It would be crazy to wait.”

Cancer Research UK, a nonprofit organization, is converting its research laboratories to conduct as many as 2,000 tests a day. But its capacity has been limited to a few hundred because of difficulty and delays in obtaining scarce materials, said Prof. Charles Swanton, its chief clinical officer.

Even the swabs used to obtain samples had turned out to be scarce, he said, and his laboratory ultimately agreed to pay a Chinese supplier as much as $6 a swab — about 100 times the typical cost. “It took about 10 days to get them,” Professor Swanton added.

The British division of the drug giant AstraZeneca began setting up a testing facility last month for its own essential workers, said Mene Pangalos, the executive overseeing the effort. But at the request of the British government, AstraZeneca and its rival drug company GlaxoSmithKline have teamed up to repurpose a laboratory at Cambridge University to carry out as many as 30,000 diagnostic tests a day by the beginning of May.

AstraZeneca hopes to develop a laboratory test for antibodies, too, Mr. Pangalos said. But that will take until at least the middle of next month, and a home-based test, such as the British government tried to order, would take much longer, he added.

“Everyone is overpromising at the moment,” he said. “I don’t want to overpromise.”