Human Rights Watch has accused the International Association of Athletics Federations of discriminating against female athletes with naturally high levels of testosterone and urged the governing body to scrap its hyperandrogenism rule.

From 1 November, female athletes who race between 400 metres and a mile and have at least five nanomoles of testosterone per litre (nmol/L) of blood will have to reduce that amount to within the usual female range for the hormone or race against men or other “intersex” athletes.

The IAAF first introduced a testosterone limit for female athletes in 2011 but the rule was suspended by the court of arbitration for sport in 2015 after an appeal by the Indian sprinter Dutee Chand.

Given two years by Cas to find a scientific justification for the rule, the IAAF produced a study last year that it claims proves female athletes with male levels of testosterone have a significant advantage over women with more usual, much lower, levels of the male sex hormone.

This advantage, the IAAF claims, is particularly marked in middle-distance events, hence the 400-metres-to-a-mile stipulation, although it has described the rule as a “living document” that can be updated as more research is carried out.

But ever since the revised rule was announced in April, the IAAF has been forced to defend itself from accusations of bad science and bias, with much of the debate focused on the rule’s most obvious target, South Africa’s two-times Olympic 800m champion Caster Semenya.

It was her arrival on the scene in 2009 that prompted the IAAF to try to find a more scientific basis for its gender rules in the first place and she remains the world’s most high-profile athlete with “differences of sexual development” (DSD).

Semenya announced her intention to challenge the rule at Cas last month and she was backed by the Women’s Sport Foundation, which accused the IAAF of “exacerbating discrimination against women in sport who are perceived as not prescribing to normative ideas about femininity”.

That view has now been supported by Human Rights Watch. In an open letter to the IAAF its women’s rights director, Liesl Gerntholtz, wrote: “The IAAF eligibility regulations for the female classification discriminate against women on the basis of their sex and their sex characteristics.

“Regulations that call for scrutiny of women’s naturally occurring hormone levels are at root a form of judgment and a questioning of women’s sex and gender identity.

“Women with intersex variations have the same rights to dignity and bodily integrity as all women. But the new IAAF regulations coerce some women to undergo unnecessary medical intervention to alter their hormone levels simply because their naturally occurring testosterone is atypical.”

The IAAF, however, has repeatedly defended its rule and the science that supports it. Last week, it responded to the Women’s Sports Foundation open letter with one of its own that said it “has not and will never try to prevent women from participating in athletics”.

It explained that women with DSD can still compete at domestic level but if they want to race the specified distances they must “take measures to ensure their testosterone levels are under 5nmol/L (which puts them on an even playing field with the rest of the female population)”.

Likening the rule to restriction on age in age-group competitions or weight divisions in combat sports, the IAAF added that it “makes no judgment about gender or sexual identity” and only wants to “maintain a fair and meaningful category for women” in athletics. PA