Swimming's world governing body FINA has voted to restrict the participation of transgender athletes in elite women's competitions.
The organisation has also created a working group to establish an "open" category for them in some events as part of its new policy.
The decision was made during FINA's extraordinary general congress on the sidelines of the world championships in Budapest on Sunday after members heard a report from a transgender task force comprising leading medical, legal and sports figures.
The new policy will require transgender competitors to have completed their transition by the age of 12 in order to be able to compete in women's competitions.
The policy was passed with a roughly 71% majority after it was put to the members of 152 national federations with voting rights who had gathered for the congress at the Puskas Arena.
Transgender rights has become a major talking point as sports seek to balance inclusivity while ensuring there is no unfair advantage.
The debate intensified after University of Pennsylvania swimmer Lia Thomas became the first transgender NCAA champion in Division I history after winning the women's 500-yard freestyle earlier this year.
It followed New Zealand weightlifter Laurel Hubbard becoming the first transgender athlete to compete at the Olympic Games in Tokyo last year.
Australian Associated Press
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