Controversial transgender swimmer Lia Thomas will not take part in the upcoming Paris Olympics after losing a legal battle in which she argued the rules prohibiting her from participating were “invalid and unlawful.”

Thomas, 25, who transitioned to female via hormone therapy after competing for three years as a male on the University of Pennsylvania swim team, dominated the field of biological women to become the first openly trans person to win an NCAA Division I title in 2022.

Her ascent sparked an impassioned national debate about whether it was fair for biologically male athletes who change their gender to compete in female sports leagues.

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Thomas filed suit against World Aquatics (WA), the world governing body for water sports including swimming, seeking to overturn their 2022 policy prohibiting athletes who have been through “any part of male puberty” from competing in the female category, The Guardian reported.

The matter was adjudicated by the Court of Arbitration for Sport (CAS), a Swiss-based international body for settling disputes that arise within the world of high-level athletics, which wrote in its 24-page decision that Thomas was “simply not entitled to engage with eligibility to compete in WA competitions.” In a statement, World Aquatics touted the ruling as “a major step forward in our efforts to protect women’s sport.”

The governing body added that it is “dedicated to fostering an environment that promotes fairness, respect, and equal opportunities for athletes of all genders and we reaffirm this pledge.”

World Aquatics’ policy was introduced after Thomas easily defeated Olympic silver medalist Emma Weyant by 1.75 seconds to claim the NCAA title in the women’s 500-yard freestyle event.

In a subsequent attempt to include as many athletes as possible, World Aquatics introduced a new “open” category inclusive of transgender swimmers, which was meant to debut at an event in Berlin last October, The Guardian noted.

However, the category was canceled after nobody signed up to enter the field.