Caster Semenya will not defend her 800m world title in Doha in September after a Swiss court reversed its suspension of the controversial ruling that would force her to take testosterone-reducing medication in order to compete in short races.

The South African's lawyers said the Swiss Federal Supreme Court reversed its prior rulings with immediate effect pending the outcome of her appeal. The U-turn came after the court had "super-provisionally" told the IAAF in that it cannot apply its rules while her appeal is ongoing.

Under the new IAAF rules, which will apply to women events between 400 metres to the mile, athletes classed as having DSDs will have to reduce their blood testosterone level to below five (5) nmol/L for a period of six months before they can compete. They must then maintain it below that level continuously.

While the IAAF says the rules are essential to preserve a level playing field, the two-time Olympic 800m champion said they are"unfair", and that she should be allowed to "run naturally, the way I was born".

Semenya has dominated the 800m race over the last decade, a period in which she became a favourite among South African athletics fans.

Semenya expressed her disappointment that she will not be able to compete in the September competition following Tuesday's development.

"I am very disappointed to be kept from defending my hard-earned title, but this will not deter me from continuing my fight for the human rights of all of the female athletes concerned," she said.

The athlete's lawyers said they would however continue to pursue the appeal.