Swimming's world governing body, FINA, on Sunday voted to restrict the participation of transgender athletes in elite women's competitions and create a working group to establish an "open" category for them in some events as part of its new policy.
The new policy will require transgender competitors to have completed their transition by the age of 12 to be able to compete in women's competitions.
The regulations would have a major impact on the career of Lia Thomas, who earlier this year became the first openly transgender woman to win a NCAA Division I women's swimming title.
Thomas declined to comment on the new policy to ESPN.
The decision was made during FINA's extraordinary general congress on the sideline of the world championships in Budapest, Hungary, after members heard a report from a transgender task force made up by leading medical, legal and sports figures.
The policy was passed with a roughly 71% majority after it was put to the members of 152 national federations with voting rights that had gathered for the congress at the Puskas Arena.
Reuters contributed to this report.