The IAAF has published rules for female athletes with high natural testosterone levels, which might force two-time Olympic 800-metre champion Caster Semenya to stop running middle-distance races.
From November 1, the IAAF will limit entry for all international events from 400m through the mile to women with testosterone levels below a specified level.
Women with elevated testosterone must reduce their level for "six months (eg: by use of hormonal contraceptives)" before being eligible to run, and maintain that lowered level.
IAAF president Sebastian Coe says track's governing body must "ensure a level playing field where success is determined by talent, dedication and hard work rather than other contributing factors."
"Our evidence and data show that testosterone, either naturally produced or artificially inserted into the body, provides significant performance advantages in female athletes."
Semenya faces taking daily medication or start racing over 5000 meters. Without the rules, the 27-year-old South African would likely defend her 800m world title next year in Doha, Qatar. She also took bronze in the 1500 at the 2017 worlds in London.
In 2011, the IAAF enacted a rule to force athletes with hyperandrogenism to artificially lower their testosterone levels to be eligible to compete.
Two years earlier, Semenya clocked a one-minute, 55-second time to win her first world title as a teenager in Berlin. While the previous rules were enforced, her season-best times were around 1:59 or slower.
The previous rules were challenged at the Court of Arbitration for Sport (CAS) by sprinter Dutee Chand of India and overturned before the 2016 Olympics. In Rio de Janeiro, Semenya retained her Olympic title, running 1:55.28.
On her Twitter account on Wednesday, Semenya did not comment but posted an image of the statement: "How beautiful it is to stay silent when someone expects you to be enraged."
The new IAAF rules could yet be challenged at CAS.
The IAAF, however, said on Thursday there was "broad medical and scientific consensus, supported by peer-reviewed data and evidence" to back its position.
"There is a performance advantage in female athletes with DSD (Differences of Sexual Development) over the track distances covered by this rule," said Dr Stephane Bermon, who works in the IAAF medical and science department.
Research over a decade showed 7.1 in every 1000 elite track and field athletes had elevated testosterone levels - 140 times greater than the female population.
"The treatment to reduce testosterone levels is a hormone supplement similar to the contraceptive pill taken by millions of women around the world," Bermon said. "No athlete will be forced to undergo surgery."
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