Athletics' governing body has denied it will tell a court female athletes with high testosterone levels like Caster Semenya should be classified as male.
World and Olympic 800m champion Semenya is challenging a proposed IAAF rule that aims to restrict the levels of testosterone in female runners.
The case will be heard at the Court of Arbitration for Sport (Cas) next week.
The Times reported that IAAF lawyers will say Semenya is a "biological male" as well as classified as female.
The IAAF said it is "not classifying" any athlete with "differences of sexual development" (DSD) - of whom South African Semenya, 28, is the most notable - as male.
"To the contrary, we accept their legal sex without question, and permit them to compete in the female category," it said in a statement on Wednesday.
"However, if a DSD athlete has testes and male levels of testosterone, they get the same increases in bone and muscle size and strength and increases in haemoglobin that a male gets when they go through puberty, which is what gives men such a performance advantage over women.
"Therefore, to preserve fair competition in the female category, it is necessary to require DSD athletes to reduce their testosterone down to female levels before they compete at international level."
The IAAF intended to bring in new rules on 1 November last year but put that back to 26 March to wait for the outcome of the legal challenge from Semenya and Athletics South Africa.
The rules will apply to women in track events from 400m up to the mile and require that athletes have to keep their testosterone levels below a prescribed amount "for at least six months prior to competing".
The delay means DSD athletes with high testosterone will not be allowed to run for six months from the date any rule change comes in, which may see Semenya miss most of the 2019 outdoor season.
The 2019 World Athletics Championships begin in Doha on 27 September.
Semenya, a two-time Olympic champion and three-time world champion, has previously been asked to undertake gender testing by athletics chiefs, but no results have officially been made public.
When she announced she would fight the rules last year, the South African said: "It is not fair. I just want to run naturally, the way I was born.
"I am Mokgadi Caster Semenya. I am a woman and I am fast."
Katrina Karkazis, a senior research fellow with the Global Health Justice Partnership at Yale University, and a leading experts in testosterone, is among those to criticise athletics' governing body over its stance.
She said the IAAF was "blithely and opportunistically misrepresenting the science of sex biology" and seeking "to create its own definition of sex based on erroneous beliefs about biology and gender".
"It is unfounded and cruel and will do even more harm by fostering misunderstanding and further discrimination," she added.