World swimming's governing body FINA on Sunday voted for new eligibility rules that restrict the participation of transgender athletes in elite women's competitions.
The policy is the strictest from any Olympic sports body and effectively bars any transgender women who have gone through male puberty from competing in women's events.
FINA also agreed to work towards establishing an "open" category for some events that would ensure all swimmers would have the chance to compete.
The decision means that swimmers such as American Lia Thomas will not be able to compete in world championships or the Olympics.
University of Pennsylvania swimmer Thomas became the first transgender NCAA champion in Division I history after winning the women's 500-yard freestyle earlier this year.
The success of Thomas, who competed on Pennsylvania's men's team for three years before transitioning and setting multiple programme records with the women's team, provoked a wide-ranging debate about the issues of inclusivity and competitive fairness in swimming and sport in general.
Here is the background to FINA's decision and why this ruling is important to the world of sport:
WHY DID FINA TAKE THIS DECISION?
There have been growing calls from former swimmers and coaches for the governing body to restrict the participation of transgender women in the sport, which intensified after Thomas's success at the U.S. college championships.
Those who campaigned for change argued that people who have gone through male puberty have physical advantages and therefore women's competition needed to be protected.
Supporters of trans participation argue that not enough research has been done into the question of whether trans women have any advantage. Groups such as Athlete Ally have stated that FINA's new policy is "discriminatory, harmful, unscientific".
Last year, New Zealand weightlifter Laurel Hubbard became the first transgender athlete to compete in the Olympics in a different gender category than assigned at birth.
Many sports bodies have allowed transgender women to compete in women's events if they have lowered their testosterone levels to a certain point.
Last week, the International Cycling Union (UCI) tightened its rules by increasing the transition period for lower testosterone from 12 months to two years and halving the maximum level to 2.5 nmol/L.
The FINA ruling could increase pressure for similar moves inside other sports.
CAN THE DECISION BE CHALLENGED?
The normal route for challenging the rulings of international sports bodies is through the Court of Arbitration for Sport, based in Lausanne, Switzerland. Other sports will be watching any legal moves with keen interest.