US transgender swimmer Lia Thomas has had a legal case against World Aquatics dismissed by the Court of Arbitration for Sport, swimming's governing body said on Wednesday.

Thomas was attempting to overturn a World Aquatics (WA) vote in 2022 that stopped transgender women from competing in women's elite races if they have gone through any stage of the process of male puberty.

The 25-year-old, who in March 22 became the first transgender athlete to win the highest level of NCAA title, attempted to argue the rules were discriminatory.

The Court of Arbitration for Sport panel found that "for the time being" Thomas is not eligible to compete in elite competitions through WA or USA Swimming.

"The Panel notes that the Athlete did not apply for - let alone was granted - the right to participate in 'Elite Events' within the meaning of the USA Swimming Policy," sport's highest court said in a 24-page verdict seen by AFP.

"She is currently only entitled to compete in USA Swimming events that do not qualify as 'Elite Events'," it added.

Therefore Thomas was "simply not entitled to engage with eligibility to compete in WA competitions" and therefore was "not sufficiently affected" by the rules to be able to challenge them, the court concluded.

World Aquatics praised the ruling, calling it "a major step forward in our efforts to protect women's sport."

"World Aquatics is dedicated to fostering an environment that promotes fairness, respect, and equal opportunities for athletes of all genders and we reaffirm this pledge," the governing body said in a statement.

"Our policies and practices are continuously evaluated to ensure they align with these core values, which led to the introduction of our open category," it added.

"We remain committed to working collaboratively with all stakeholders to uphold the principles of inclusivity in aquatic sports and remain confident that our gender inclusion policy represents a fair approach."