One of the world’s leading governing bodies for water sports has banned the vast majority of trans female athletes from competition following the release of new inclusion guidelines. LGBTQ+ advocacy organzations have expressed disappointment in the decision, saying that it will increase discrimination against an already marginalized group of athletes.

A 24-page “gender inclusion” policy released by the Fédération Internationale De Natation (FINA) on June 19 prohibits trans women from competing in sports like swimming, diving and water polo if they underwent endogenous male puberty. In order to be able to participate in alignment with their gender identity, trans female athletes will be required to have begun transitioning at the age of 12 or prior to reaching what’s called Tanner Stage 2, that is, the point at which AMAB adolescents begin to develop pubic hair.

FINA, which oversees over 200 national federations on six continents, said in its new guidelines that the policies are necessary to adjust for the “performance gap” that becomes apparent when young athletes reach the age of puberty.

“Without eligibility standards based on biological sex or sex-linked traits, we are very unlikely to see biological females in finals, on podiums or in championship positions; and in sports and events involving collisions and projectiles, biological female athletes would be at greater risk of injury,” the guidelines state.

FINA’s president, Husain Al-Musallam, added that the rules were intended to ensure that all athletes are able to compete on a level playing field during events.

“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,” he said in a statement. “FINA will always welcome every athlete.”

The declaration will make it extremely difficult for trans female athletes like University of Pennsylvania swimmer Lia Thomas to qualify for the Olympics, as FINA determines eligibility for the quadrennial games. In March, the International Olympic Committee (IOC) released an updated policy allowing individual sport governing bodies like FINA to develop their own guidelines on trans inclusion.

Thomas had expressed interest in becoming the first out trans female swimmer to compete in the Olympics after shattering women’s records in the 500-yard freestyle this year. She became the first trans athlete in history to win an NCAA swimming championship, sparking criticism that she has an unfair advantage.

The topic of a competitive advantage among trans women in sports has been under-researched, leading the IOC to call for more scientific inquiry into the topic. At the time of publication, researchers have yet to demonstrate that trans women who are taking medications like hormone replacement therapy (HRT) or puberty blockers have an ingrained biological edge over their cis counterparts.

But despite the lack of information on the subject, at least 18 U.S. states have passed laws restricting trans athletes from competing in alignment with their gender identity at the K-12 or collegiate level. In April, U.K. prime minister Boris Johnson also called for a trans athlete ban in Britain.

Amid the growing backlash to trans inclusion in sport, athletics associations like World Rugby and International Cycling Union (UCI) have rolled out policies making it harder for trans athletes to participate according to their lived gender.

Critics maintain these policies are discriminatory. Following the release of FINA’s new guidelines, Anne Lieberman, director of policy and programs at the LGBTQ2S+ sports advocacy organization Athlete Ally, said in a statement that preventing trans female athletes from competing alongside cis women is “harmful” and “unscientific.”

“The eligibility criteria police the bodies of all women and will not be enforceable without seriously violating the privacy and human rights of any athlete looking to compete in the women’s category,” Lieberman said.

The Human Rights Campaign (HRC), America’s largest LGBTQ2S+ rights group, added in a press release that the FINA policies are a “blatant attack on transgender athletes who have worked to comply with long-standing policies that have allowed them to participate for years without issue.”

“This policy is an example of swimming organizations caving to the avalanche of ill-informed, prejudiced attacks targeted at one particular transgender swimmer,” said Joni Madison, HRC’s interim president. “We urge the FINA to rethink its policy and ensure inclusion for all athletes—including transgender women—and allow them to participate in sports free from discrimination, abuse and harassment.”

FINA has maintained that the guidelines were developed in consultation with a working group that included trans athletes and coaches, in addition to scientific, legal and human rights experts.

The governing body has also signalled that it intends to develop an “open competition” category for trans athletes who may otherwise be banned from events. Al-Musallam said that the first-of-its-kind category would “mean that everybody has the opportunity to compete at an elite level,” although FINA has yet to determine how it would work.

“This has not been done before, so FINA will need to lead the way,” he said. “I want all athletes to feel included in being able to develop ideas during this process.”