A rule to limit testosterone levels in female athletes would be an 'unscientific' way of trying to level the field, experts have warned.
The International Association of Athletics Federations (IAAF) is planning to introduce a new limit amid a raging debate about how transgender women should compete.
But two scientists argue a person's sex is not determined solely by the amount of testosterone – the male sex hormone – in their body.
They warn focusing on this could overlook other genetic advantages of men, and that there is some crossover in the varying testosterone levels among athletes.
Another expert added the ruling appears to be 'personal' and unfairly targeted at champion runner Caster Semenya, who is intersex and has challenged the rule in court.
Runner Caster Semenya, 28, was subjected to a sex test when people claimed she was a man after testing found higher than usual levels of testosterone in her blood. She was temporarily banned from competing as a woman but has since been allowed to return
The IAAF's proposed rule has been postponed after the legal challenge from South African Ms Semenya, who has been banned from competing before.
She was found to have testosterone levels above the female threshold – at the time the IAAF limit was 10nmol/L – and was subjected to a sex verification test.
She is believed to be intersex, meaning she is biologically both sexes – she is said to be a woman but have no womb or ovaries but internal testicles instead.
The testosterone issue also involves transgender athletes, with concerns about biological men competing as women without having sex-change surgery.
Currently it is possible for trans athletes to compete while taking hormonal therapy to reduce their testosterone levels but to still have male sex organs.
Leading sportswomen including Dame Kelly Holmes, Paula Radcliffe, swimmer Sharron Davies and tennis player Martina Navratilova all say women's sport could be compromised by former men being allowed to compete.
Writing in the British Medical Journal today, two experts argue testosterone monitoring may not be accurate enough.
Sheree Bekker, from the University of Bath, and Cara Tannenbaum, from the University of Montreal, said testosterone levels vary naturally.
Among track and field elite athletes, they said, there is known to be a crossover between men with low levels and women with high levels.
Under new IAAF rules, women will not be allowed to compete in running races if they have more than 5nmol/L of testosterone in their blood.
Martina Navratilova has said it is unfair that a transgender woman can compete as a female by just lowering testosterone levels with drugs and that they do not need to have sex-change surgery. She said: 'It's insane and it's cheating'
Those who do, and qualify as female or intersex, will have to use hormonal drugs to lower their testosterone.
But Dr Tannenbaum and Dr Bekker said a link between testosterone levels and medal-winning ability has never been proven.
And they say there are other ways women could have biological advantages even if their testosterone levels are within the limit – although these are not explained.
Two high profile cases of women competing with claimed biological advantages have been those of the runner Mokgadi Caster Semenya, and Rachel McKinnon.
Caster Semenya, 28, was banned from running for almost a year in 2009 after being found to have higher than usual levels of testosterone in her body.
Critics claimed she was a man which led to her being subjected to a sex test.
The results were never published but are believed to have
