(CNN) Runner Caster Semenya will not be able to compete internationally without taking testosterone-reducing medication, after a Swiss judge reversed an earlier ruling on Monday.

The new decision will prevent the South African athlete from defending her 800-meter title at the World Championships in Doha, Qatar in September.

According to a policy recently devised by the International Association of Athletics Federations (IAAF), all athletes with a difference in sex development should reduce their blood testosterone to a specific level for at least six months -- and then maintain that for the rest of their athletic career -- in order to compete in events from 400 meters to a mile.

The 28-year-old track star, who has naturally elevated levels of testosterone, has remained "steadfast in her defiance" of rule, according to a statement released by her representative today.

A Swiss court had temporarily suspended the controversial new policy in June. If the suspension had continued, Semenya would have been allowed to compete in Doha while her appeal to have the new rules reversed is ongoing.

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