Sports journalism has rarely concerned itself with women’s sport. Until recently. BBC Wales profiled Kelly Morgan, a player for Porth Harlequins Ladies rugby team: a “trailblazer” who plays “with a smile on my face”. The Times wrote about Maxine Blythin of the Kent women’s cricket side, whose batting average this season is a Bradmanesque 124. New Zealander Laurel Hubbard (pictured) gained attention for scooping the women’s weightlifting gold in the Pacific Games, aged 41. Andraya Yearwood and Terry Miller, high-school runners in Connecticut, are winning medals in state and regional competitions. June Eastwood, a cross-country runner at the University of Montana, is talked of as a future national champion.

These stories are less exceptional at a second look: every one of these athletes was born male. They are part of a wave of trans women entering women’s sport, something enabled in 2015, when the International Olympic Committee (IOC), the de facto global standard-setter for sport, stopped requiring transgender women—people born male who identify as women—to undergo sex reassignment surgery before entering women’s events. Instead, they merely need to reduce their testosterone for a year below a certain level. As their number increases, it is becoming evident that sporting bodies have made a grave mistake.

Rugby is a sharp illustration of the risks to women’s safety. As Heyneke Meyer, a former coach of South Africa’s national team, once said: “Ballroom dancing is a contact sport; rugby is a collision sport.” Morgan, the Welsh rugby player, is “going to be a good, good player for the next few years, as long as we can stop her injuring players in training,” her coach quipped to the BBC. The club captain laughingly recalled Morgan, who is six foot and works as a lorry driver, folding a female player “like a deckchair”.

Trans women are relegating female competitors to also-rans. Blythin offers a handy like-for-like comparison: playing for a lower-ranked men’s club, Blythin’s batting average was a ho-hum 15. Hubbard’s victory at the Pacific Games deprived Feagaiga Stowers, a Samoan teenager who had escaped domestic abuse and sexual violence, of a gold medal. “It’s not easy for the female athletes to train all year long to compete and yet we allow these stupid things to happen,” the Samoan prime minister complained. Yearwood and Miller are being courted by talent scouts from big-name American colleges offering scholarships that would otherwise have gone to females. Eastwood’s times suggest a women’s world-record-breaker in the making.

Until recently sex-segregated sports were no more controversial than separate competitions for under-18s, or weight categories in boxing. Female and male bodies differ in many ways that make the best women uncompetitive with even mediocre men. The differences, including greater average height and weight, stronger bones, larger muscles with more fast-twitch fibres, and bigger hearts and lungs, are mostly caused by the rush of testosterone during male puberty. According to Dr Emma Hilton, a developmental biologist, few of these differences are reversible even if testosterone is later suppressed.

The IOC’s fateful decision was made under pressure of a sustained campaign to persuade governments, under the rubric of trans rights, to recast the legal categories of “men” and “women” to depend not on objective, immutable sex but on subjective, self-declared gender. Already some American states and Australian provinces, plus Canada, Ireland, New Zealand and South Africa, have gone down this path. The British government is considering doing the same, and in the meantime the shift to gender self-identification has in practice largely happened. People with knowledge of the IOC’s workings say that it was threatened with legal action if it did not lower the barriers to trans women competing in women’s sport.

According to Ross Tucker, a sports scientist and consultant for World Rugby, the IOC looked for a way that would allow trans women to compete without their pre-existing male advantage. It seized gratefully upon a study by a trans woman, Joanna Harper, purporting to show that a modest reduction in testosterone pretty much removed the male sporting advantage. Harper asked eight recreational runners, all trans women, to recall how fast they had run before they had started to take cross-sex hormones, and concluded that their speeds had fallen by 10-12 per cent. The IOC committee that decided, on this slender basis, to open up women’s sports to males contained not a single woman.

Better science suggests that the IOC got it terribly wrong. Research teams in Sweden and South Africa are tracking transitioning male athletes undergoing testosterone suppression—and the early results suggest that after a year muscle mass falls by just a few per cent, and muscle strength does not fall at all. In other words, their male advantage is retained. That advantage is so great that despite their tiny numbers trans women will soon dominate podiums across women’s sports and set records that no female can hope to beat.

Fair Play for Women, a campaign group, is calling on sports bodies and governments to reverse course on grounds of both safety and fairness. Right now, it is looking for female rugby players to join a test case. It wants to argue that the risks posed by trans inclusion are so great that natal women will be forced to self-exclude, which amounts to indirect sex discrimination. A personal injury claim could provide another avenue for a legal challenge—but first a woman would have to be seriously hurt. UK courts have found rugby bodies liable for causing unreasonable risks by failures to apply player-safety laws, says Tim O’Connor, an Irish barrister specialising in sports injuries. Re-excluding players born male from women’s rugby on the ground that their inclusion had introduced unreasonable risks would have a good chance of withstanding the inevitable legal challenges, he thinks.

But to face down trans activists, who smear anyone who dares to mention the physical differences between males and females as bigoted and transphobic, would take courage. Up till now, sporting bodies have chosen instead to fold—like deckchairs.