Some female runners with naturally high testosterone levels will have to race against men or change events unless they take medication, under new rules issued by athletics' governing body.
The rules, which start on 1 November 2018, would apply to women who race in track events from 400m up to the mile.
The IAAF believes the new measures will stop women with high testosterone levels gaining a competitive advantage.
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"The revised rules are not about cheating - they are about levelling the playing field to ensure fair and meaningful competition," IAAF president Lord Coe said.
"We want athletes to be incentivised to make the huge commitment and sacrifice required to excel in the sport."
Athletics chiefs have been compared to apartheid-era South African leaders after announcing new rules governing testosterone levels in female athletes.
The changes mean some female runners with naturally high testosterone levels will have to race against men or take medication if they wish to compete.
South Africa's Olympic 800m champion Caster Semenya could be affected.
And her country's ruling party, the African National Congress, says the measures are "blatantly racist".
It wants the South African government to challenge the rule change - which comes into effect in November - at the Court of Arbitration for Sport (Cas).