Athletics' governing body the IAAF has ruled some female runners with naturally high testosterone levels will have to race against men, or change events, unless they take medication.

Semenya will fight the ruling at the Court of Arbitration for Sport (Cas).

"It is not fair. I just want to run naturally, the way I was born," said the 27-year-old South African.

"I am Mokgadi Caster Semenya. I am a woman and I am fast."

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The rule, which comes into force on 1 November, applies to women who race in track events from 400m up to the mile.

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 ever officially been made public.

Testosterone is a hormone that increases muscle mass, strength and haemoglobin, which affects endurance.

Lawyers Norton Rose Fulbright are leading Semenya's legal challenge in Lausanne.

They said: "Ms Semenya, like all athletes, is entitled to compete the way she was born without being obliged to alter her body by any medical means."

Cas says an arbitration procedure has been opened and a hearing will be held at a later date.