•FINA's decision, the strictest by any Olympic sports body, came after members heard a report from a transgender task force comprising medical, legal and sports figures.
•With the world championships currently taking place in Budapest, FINA was not immediately available for comment.
Transgender cyclist Veronica Ivy questioned the move by swimming's governing body FINA to restrict the participation of trans athletes in elite women's competitions, telling Reuters that there had not been enough research to guide such decisions.
Ivy also criticised FINA's plans to explore an "open" category as part of its policy that was passed following a vote at its extraordinary general congress, saying such a move does not show respect to trans athletes.
FINA's decision, the strictest by any Olympic sports body, came after members heard a report from a transgender task force comprising medical, legal and sports figures.
The new eligibility policy for FINA competitions states that male-to-female transgender athletes are eligible to compete only if "they have not experienced any part of male puberty beyond Tanner Stage 2 (of puberty) or before age 12, whichever is later."
Canadian Ivy, who in 2018 became the first transgender UCI masters world track cycling champion by winning in the women's 35-44 years category, described FINA's policy as "unscientific".
"There hasn't been a single peer-reviewed study on trans women competitive swimmers to show that there's any competitive advantage for transitioning post-puberty," Ivy said.
"So to single out puberty as the break-off point isn't based on any evidence, it's not based on them seeing an advantage for trans women, they have only looked at cisgender male athletes compared to cisgender female athletes.
"That's not how this works. When you're trying to single out trans women, you need to study trans women athletes... FINA has not done that."
With the world championships currently taking place in Budapest, FINA was not immediately available for comment.
But FINA's contention that trans women athletes retain some physical advantages despite testosterone suppression after going through male puberty has been backed by a study from Joanna Harper, a doctoral researcher at Loughborough University.
That paper concluded that "strength may be well preserved" in trans women during the first three years of hormone therapy.
"It's reasonable to put restrictions on trans women in international level sports competitions," Harper said, adding she was disappointed but not surprised by FINA's policy.
"Restricting participation to only those trans women who have never experienced male puberty is not necessary to ensure meaningful competition for all women.
"In terms of the open category, I'm skeptical, but I'll reserve judgment until I see the finished product. Will there be Olympic medals in swimming in the open category? Will swimmers in that category be able to earn a living as professionals?"
The debate surrounding trans women athletes has 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 this year.
The rugby league also banned transgender players from women's international competitions on Tuesday.
Soccer's global body Fifa is in a consultation process over transgender participation while World Athletics chief Sebastian Coe has praised FINA for its stance.
The International Cycling Union has also tightened its rules on transgender participation.
Ivy said that FINA's decision was likely because of Thomas, who has not won a world title or participated in the Olympics.
"In the United States trans people are under attack... People trying to outlaw treatments and access to healthcare and sport... politicians calling for us to be murdered," she said.
"That's the context in which we see something like this."
Harper said it was important for sports bodies to continue collecting data.
"I'd suggest that whatever decisions they make today, they review them regularly and hopefully, as more and better data is published, they make better policies," Harper said.