A landmark report in the UK has confirmed that, on average, trans female athletes retain some competitive advantages compared to cis-gendered women.
The Sports Councils Equality Group (SCEG), which covers UK Sport, Sport England, Sport Wales, Sport Scotland and Sport Northern Ireland, has released its findings after an 18-month investigation.
The Group ruled that trans athletes, fairness and safety cannot 'co-exist in a single competitive model'.
SCEG has recommended introducing an 'open' category at competitions up to the national level for trans athletes to compete in instead of male or female.
Their guidance only covers community-level sport and doesn't apply to international, professional or elite sport.
"In order to survive and thrive in the future, sport must adapt to reflect modern society, and that often, it is too slow to do so," the guidance said.
Their investigation concluded that 'testosterone suppression is unlikely to guarantee fairness between transgender women and natal females in gender-affected sports' and that there are 'retained differences in strength, stamina and physique between the average woman compared with the average transgender woman or non-binary person registered male at birth'.
Each individual sport is now urged to look at the regulations they have in place to see if they need to be updated.
Some codes will need to prioritise safety if their sport involves contact, collision or combat, while others will have to look at fairness if they're based on strength, stamina or physique.
The report also noted that a 'case-by-case' system of assessment wouldn't be 'practical nor verifiable for entry into gender-affected sports'.
Reaction to the report has been mixed.
Joanna Harper, who worked as an adviser to the Olympic Games on the inclusion of trans athletes, told Sky News there are some issues with the SCEG report.
"The suggestion that the maintained advantages of trans women preclude equitable competition or safety in women's sports is questionable at best," she said.
"There is much research yet to do before we can quantify the maintained advantages of trans athletes, but trans women have been competing in women's sports for more than 40 years and are still severely under represented.
"Trans women are not on the verge of taking over women's sport."
Fair Play For Women's Dr Nicola Williams countered with: "It is now increasingly recognised that the existing approach to transgender inclusion in sport is out of date and no longer fit for purpose.
"We commend the Sports Councils for taking the lead to address this difficult and sensitive issue.
"This comprehensive review confirms what we all know: sex matters in sport. That's why we have always needed a separate protected category for females and still do.
The Olympics has its own set of requirements for trans athletes to compete in, however the head of the International Olympic Committee admitted after the 2020 Tokyo Games that their system might need to be revised.
New Zealand's Laurel Hubbard made history this year when she became the first trans athlete to qualify and compete at an Olympic Games.
She participated in the women's +87kg weightlifting event, however didn't lift enough to claim a medal.
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