World Athletics president Sebastian Coe has urged care on deciding transgender participation rules in women's sports, which he says have a "very fragile" future while "gender cannot trump biology".
Coe made his comments to The Times amid controversy at the weekend, when University of Pennsylvania swimmer Lia Thomas became the first transgender athlete to claim the highest US national college title by winning the women's 500-yard freestyle in Atlanta.
"I think that the integrity of women's sport if we don't get this right, and actually the future of women's sport, is very fragile," Coe said, with the National Collegiate Athletic Association (NCAA) handing over the responsibility to individual bodies to determine their own policies for trans athletes in line with the International Olympic Committee's (IOC) stance.
"There is no question to me that testosterone is the key determinant in performance," Coe went on, as USA Swimming updated its rules for elite swimmers in February which will see testosterone tests for trans athletes 36 months before competitions in a bid to reduce any unfair advantages.
At World Athletics, transgender athletes must prove low testosterone levels across a 12-month period to be allowed to take part in their events.
"Look at the nature of 12 or 13-year-old girls," Coe suggested. "I remember my daughters would regularly outrun male counterparts in their class but as soon as puberty kicks in that gap opens and it remains. Gender cannot trump biology," he claimed.
"You can't be oblivious to public sentiment, of course not. But science is important," Coe stressed. "If I wasn't satisfied with the science that we have and the experts that we have used and the in-house teams that have been working on this for a long time, if I wasn't comfortable about that, this would be a very different landscape."
Meanwhile one of Thomas' rivals has spoken out against the UPenn swimmer, who was on the Ivy League college's men's team for three years prior to beginning hormone replacement therapy in 2019.
In a letter sent to the leading college sports organization, Hungary's Reka Gyorgy accused the NCAA of denying her a "spot in the final" in one of its swimming championships through allowing Thomas to compete.
"It hurts me," Gyorgy allegedly wrote, after failing to make the consolation final by one spot in her last ever college meet with a 17th place showing in the preliminary races for the 500-yard freestyle event, which also left her feeling "frustrated".
"It feels like that final spot was taken away from me because of the NCAA's decision to let someone who is not a biological female compete," said the 25-year-old, who starred at the Rio 2016 Olympic Games and has featured on Virginia Tech's swimming team for the past five years.
While Gyorgy noted that Thomas was "doing what she is passionate about and deserves that right", Gyorgy said she wanted "to critique the NCAA rules that allow her to compete against us".
"I'm writing this letter right now in hopes that the NCAA will open their eyes and change these rules in the future," Gyorgy claimed. "It doesn't promote our sport in a good way and I think it is disrespectful against the biologically female swimmers who are competing in the NCAA."
Not everyone is against Thomas' participation, however, with Olympic silver medalist Erica Sullivan shaking hands with Thomas after the 500-yard freestyle race and writing a Newsweek piece supporting Thomas' inclusion on the eve of the meeting.
"Like anyone else in this sport, Lia has trained diligently to get to where she is and has followed all of the rules and guidelines put before her," said Sullivan, who finished third while 400m individual medley silver medalist Emma Weyant from Virginia claimed second.
"Like anyone else in this sport, Lia doesn't win every time. And when she does, she deserves, like anyone else in this sport, to be celebrated for her hard-won success, not labeled a cheater simply because of her identity," Sullivan demanded.
"As a woman in sports, I can tell you that I know what the real threats to women's sports are: sexual abuse and harassment, unequal pay and resources and a lack of women in leadership," she also said.
Yet neither Coe's nor Gyorgy's comments should affect Thomas, who revealed after her win that she tries to "ignore" such rhetoric "as much as I can".
"I try to focus on my swimming, what I need to do to get ready for my races, and just try to block out everything else," Thomas added.