WASHINGTON — A highly infectious variant of the coronavirus that was first identified in Britain has become the most common source of new infections in the United States, the director of the Centers for Disease Control and Prevention said on Wednesday. The worrisome development comes as officials and scientists warn of a possible fourth surge of infections.

Federal health officials said in January that the B.1.1.7 variant, which began surging in Britain in December and has since slammed Europe, could become the dominant source of coronavirus infections in the United States, leading to a huge increase in cases and deaths.

At that point, new cases, hospitalizations and deaths were at an all-time high. From that peak, the numbers all declined until late February, according to a New York Times database. After several weeks at a plateau, new cases and hospitalizations are increasing again. The average number of new cases in the country has reached nearly 65,000 a day as of Tuesday, concentrated mostly in metro areas in Michigan as well as in the New York City region. That is an increase of 19 percent compared with the figure two weeks ago.

Dr. Rochelle Walensky, the C.D.C. director, who warned last week that she felt a recurring sense of “impending doom,” said on Wednesday that 52 of the agency’s 64 jurisdictions — which include states, some major cities and territories — are now reporting cases of these so-called “variants of concern,” including B.1.1.7.

60 percent more contagious and 67 percent more deadly than the original form of the coronavirus, according to the most recent estimates. The C.D.C. has also been tracking the spread of other variants, such as B.1.351, first found in South Africa, and P.1, which was first identified in Brazil.

The percentage of cases caused by variants is clearly increasing. Helix, a lab testing company, has tracked the relentless increase of B.1.1.7 since the beginning of the year. As of April 3, it estimated that the variant made up 58.9 percent of all new tests.

That variant has been found to be most prevalent in Michigan, Florida, Colorado, California, Minnesota and Massachusetts, according to the C.D.C. Until recently, the variant’s rise was somewhat camouflaged by falling infection rates over all, leading some political leaders to relax restrictions on indoor dining, social distancing and other measures.

against the warnings of some scientists.

Federal health officials are tracking reports of increasing cases associated with day care centers and youth sports, and hospitals are seeing more younger adults — people in their 30s and 40s who are admitted with “severe disease,” Dr. Walensky said.

It is difficult for scientists to say exactly how much of the current patterns of infection are because of the growing frequency of B.1.1.7.

“It’s muddled by the reopening that’s going on and changes in behavior,” said Dr. Adam Lauring, a virologist at the University of Michigan.

But he noted that people were becoming less cautious at a time when they should be raising their guard against a more contagious variant. “It’s worrisome,” he said.

At the same time, the United States is currently vaccinating an average of about three million people a day, and states have rushed to make all adults eligible. The C.D.C. reported on Tuesday that about 108.3 million people had received at least one dose of a Covid-19 vaccine, including about 64.4 million people who have been fully vaccinated. New Mexico, South Dakota, Rhode Island and Alaska are leading the states, with about 25 percent of their total populations fully vaccinated.

Scientists hope that vaccination will blunt any potential fourth surge.

On Tuesday, President Biden moved up his vaccination timetable by two weeks, calling states to make every American adult eligible by April 19. All states have already met or expect to beat this goal after he initially asked that they do so by May 1.

hundreds of genomes predicted that this variant could become predominant in the country in a month. At that time, the C.D.C. was struggling to sequence the new variants, which made it difficult to track them.

But those efforts have substantially improved in recent weeks and will continue to grow, in large part because of $1.75 billion in funds for genomic sequencing in the stimulus package that Mr. Biden signed into law last month. By contrast, Britain, which has a more centralized health care system, began a highly promoted sequencing program last year that allowed it to track the spread of the B.1.1.7 variant.

“We knew this was going to happen: This variant is a lot more transmissible, much more infectious than the parent strain, and that obviously has implications,” said Dr. Carlos del Rio, a professor of medicine and an infectious disease expert at Emory University. In addition to spreading more efficiently, he said, the B.1.1.7 strain appears to cause more severe disease, “so that gives you a double whammy.”

Perhaps even more troubling is the emergence of the virulent P.1 variant in North America. First identified in Brazil, it has become the dominant variant in that country, helping to drive its hospitals to the breaking point. In Canada, the P.1 variant emerged as a cluster in Ontario, then shut down the Whistler ski resort in British Columbia. On Wednesday, the National Hockey League’s Vancouver Canucks said at least 21 players and four staff members had been infected with the coronavirus.

“This is a stark reminder of how quickly the virus can spread and its serious impact, even among healthy, young athletes,” the team’s doctor, Jim Bovard, said in a statement.