Anew policy barring most trans women from elite swimming has sent shockwaves through sport this week. Within a day, the International Rugby League announced that it would be barring trans women from international competition, and football’s governing body, Fifa, stated that it would be also be reviewing its gender eligibility regulations. Those who support the inclusion of trans athletes are in despair, and those who say they should be excluded are jubilant. But as a discrimination barrister, I know these policies don’t appear in a vacuum. Are they compliant with UK law? And could they be vulnerable to legal challenge? Both questions are key – especially when considering whether other sports will follow suit. The policy announced by Fina, the international body governing water sports, including swimming, diving and water polo, is significantly more stringent than its previous regime. Under the old rules, trans women competitors had to maintain levels of bodily testosterone within the female range for an extended period. Yet the new policy rules that women cannot participate in the female category if they have undergone any aspect of male puberty. This will have come as a devastating blow to those aquatic athletes who have been following the previous regime. The suggestion in the policy that they are encouraged to stay involved with aquatics through coaching, officiating or administration is unlikely to offer much comfort. Fina has justified its decision by pointing to its “science group” of advisers, who state: “Biological sex is a key determinant of athletic performance with males outperforming females in sports (including aquatic sports) that are primarily determined by neuromuscular, cardiovascular, and respiratory function, and anthropometrics including body and limb size.” As a result, they argue, it is necessary for trans women who have experienced even partial male puberty to be excluded – and for trans men to acknowledge the risks they face in competing in the male category. Fina has reached its decision after consultation with aquatic athletes, sports scientists and legal experts, and on the face of it the new policy is consistent with the International Olympic Committee framework published in November 2021, which encouraged all international sports federations to develop eligibility criteria. It also appears superficially compliant with UK equality law, which permits the exclusion of transgender competitors in gender-affected sport on the grounds of either safety or fair competition. It seems likely that other sports federations will follow suit and Sebastian Coe has hinted that international athletics, governed by World Athletics, may tread the same path. Look closer, however, and the legal situation is less clearcut. Given the difficulties increasingly experienced by trans people in accessing gender-affirming healthcare before puberty, this is likely to be a complete ban – if transition cannot occur before puberty, then no trans athletes will qualify under Fina’s rules. This means that a challenge based on human rights legislation and the exclusion of an entire group from elite sport is possible in national courts, or at the court of arbitration for sport in Switzerland. A focus of such a challenge might be the IOC guidance on which the Fina policy is said to be based, which states that there should not be an assumption of male advantage in sports. Aspects of the science in this area are hotly debated and it is not clear which experts participated in Fina’s “science group”, or which scientific studies they have relied upon to form their view. These matters would have to be made clear if any challenge was brought. The former Canadian Olympic swimmer, now New York human rights lawyer, Nikki Dryden, has also been critical of the process used by Fina to obtain the 71% vote among members to initiate the policy. She says that member organisations only had 14 minutes to scrutinise the 24-page policy before voting on it. Other sporting organisations will need to consider carefully whether any alleged sex-based advantage applies in the particular circumstances of their sport. In football, for example, being smaller and nimbler might balance out advantages of absolute speed or strength. The trans cyclist Emily Bridges, recently excluded from elite competition by British Cycling, explained in an interview with ITV that the reduced testosterone regime can leave a trans athlete with a larger male frame but only female-level cardiovascular and respiratory performance – thereby putting them at a disadvantage. Lastly, it must be remembered that this ruling only applies to elite sport. At less exalted levels, inclusion may be of much greater importance than absolute competitive fairness. After all, sport is often not “fair” in the sense that competition favours those of certain body types – for example, the high jump tends to select for tall, thin athletes and discus throwing for those able to produce explosive power. In the months ahead, more sports organisations are likely to come out with their own eligibility criteria – and sports lawyers will probably be busy, too, as challenges to them emerge. Robin Moira White is a discrimination barrister at Old Square Chambers, London, and joint author of A Practical Guide to Transgender Law