The world governing body for swimming effectively barred transgender women from the highest levels of women’s international competition on Sunday, intensifying a debate over gender and sports that has roiled state legislatures and increasingly divided parents, athletes and coaches at all levels.

The vote by FINA, which administers international competitions in water sports, prohibits transgender women from competing unless they began medical treatments to suppress production of testosterone before going through one of the early stages of puberty, or by age 12, whichever occurred later. It establishes one of the strictest rules against transgender participation in international sports. Scientists believe the onset of male puberty gives transgender women a lasting, irreversible physical advantage over athletes who were female at birth.

World swimming would also establish a new, “open” category for athletes who identify as women but do not meet the requirement to compete against people who were female at birth.

More than 70 percent of FINA’s member federations voted to adopt the policy, which was devised by a working group set up in November that included athletes, scientists and medical and legal experts. The policy will go into effect Monday, just days after the start of the world swimming championships in Budapest.

“We have to protect the rights of our athletes to compete, but we also have to protect competitive fairness at our events, especially the women’s category at FINA competitions,” Husain al-Musallam, the president of the federation, said in statement.

There are no transgender women competing at the world swimming championships.

The move, however, came just three months after Lia Thomas became the first transgender woman to win an N.C.A.A. Division I swimming championship — she won the 500-yard women’s freestyle — putting a spotlight on the issue. She has said little about her win, but recently told Sports Illustrated: “I’m not a man. I’m a woman, so I belong on the women’s team.” She has also said that she hopes to try to qualify for the U.S. Olympic team in 2024. Under the new rule, she would not be eligible to compete there.

During Thomas’s championship season, U.S.A. Swimming, the sport’s national governing body, passed new rules that allowed transgender women to compete if they had taken medication that had sufficiently suppressed their testosterone levels continuously for 36 months. The rule put the organization in line with numerous other governing bodies, from track and field to cycling, that have relied on measuring testosterone levels to determine who gets to compete against athletes who were female at birth.

FINA’s rules apply only to international competitions but could guide the thinking of other sports federations dealing with the issue.

Advocates on both sides of the issue said the international swimming body’s move could advance the growing movement to prevent transgender women from competing even in recreational sports while undercutting efforts to provide full access to sports for people regardless of the gender they were assigned at birth.

Sports federations have sought to produce policies that try to balance science with fair play, inevitably angering people at every level of sport.

“It’s very unfortunate that FINA has made this ruling,” said Joanna Harper, a medical physicist who has written extensively on gender and sport and advised several international sports federations, including the International Olympic Committee. “Trans women are not taking over women’s sports, and they are not going to.”

Alejandra Caraballo, an instructor at Harvard Law School and an expert on transgender issues, said the FINA rule would give the green light for other bodies to pass similarly restrictive bans and also require athletes to produce as many as 10 years or more of invasive medical records and blood tests.

“This is an incredibly discriminatory policy that is attempting to fix a problem that doesn’t exist,” Caraballo said, adding, “This is the result of a moral panic because of Lia Thomas.”

Anne Lieberman, the director of policy and programs at Athlete Ally, a group that supports the rights of L.G.B.T.Q. athletes, called the regulation “deeply discriminatory, harmful, unscientific” and out of step with the I.O.C.’s guidelines on fairness and inclusion. They said the regulations would “not be enforceable without seriously violating the privacy and human rights of any athlete looking to compete in the women’s category.”

Athletes can appeal the ruling at the Court of Arbitration for Sport, the world’s top sport court, which is based in Switzerland.

The court has in the past upheld the rights of international sports federations to set rules on the classification of athletes based on their testosterone levels. In 2019, the court upheld the world track and field federation’s regulations on athletes born with both male and female genitalia that had prohibited them from competing unless they took medication to suppress their testosterone levels.

In November, the International Olympic Committee ceded eligibility rules in women’s sports to the governing bodies of individual sports. But it also said that “until evidence determines otherwise, athletes should not be deemed to have an unfair or disproportionate competitive advantage due to their sex variations, physical appearance and/or transgender status.”

FIFA, soccer’s world governing body, this year said that it would continue to allow transgender women to compete. World Rugby has barred transgender women from international competitions since October 2020.

Peer-reviewed studies show that even after testosterone suppression, top-level transgender women retain a substantial edge when racing against top biological women, according to Michael J. Joyner, a doctor at the Mayo Clinic in Rochester, Minn., who studies the physiology of male and female athletes.

Men on average have broader shoulders, bigger hands, longer torsos, greater lung and heart capacity and their muscles are denser.

“There are social aspects to sport, but physiology and biology underpin it,” Dr. Joyner said in an interview with The New York Times this year.

Ross Tucker, a sports physiologist who consults with World Rugby, has called Thomas the perfect symbol for what can happen if sports allow transgender women to compete with no restrictions.

That said, there has been relatively little scientific study of elite transgender athletes. And while testosterone’s role in physical strength and stamina is robust, studies have not been able to gauge its precise impact on performance.

Last year, World Athletics, the world governing body for track and field, which has imposed strict restrictions on runners who compete in some women’s events, corrected its own research, acknowledging that it could not find a causal relationship between high testosterone levels and enhanced athletic performance among elite female athletes.

It is not clear whether Thomas’s performance prompted FINA to pass such a strict rule about participation.

Thomas, who competed for the University of Pennsylvania, was lauded by supporters as a brave and courageous athlete this year, but her performance also sparked a backlash. Members of her own team complained about her participation, and a group of swimmers at Princeton went to the commissioner of the Ivy League to lobby against her.

Several states have passed laws barring transgender women from competitions. Some states, including Texas, have moved to outlaw medical intervention for transitioning for younger children, which would disqualify them from meeting the FINA regulations.

The question now is whether the swimming federation’s strict ruling will have a trickle-down effect. Harper, who supports sport-specific restrictions on testosterone levels for transgender women at the international level, said she was concerned that even local organizations would feel justified in barring transgender athletes. She citied the recent case of a 60-year-old transgender woman who was prevented from participating in a lawn bowling competition.

“At the international level, there is some logic to do this,” she said of the restrictions. The danger, she said, is that the people making these decisions at the recreational level “will look at FINA and put these regulations on middle school kids.”

Billy Witz, Jeré Longman, Azeen Ghorayshi and Livia Albeck-Ripka contributed reporting.