Lia Thomas has launched legal action against World Aquatics as the American swimmer bids to compete again in elite female sport.

Thomas, who is transgender, has been unable to do so since a ruling by swimming’s governing body two years ago that blocked transgender female athletes from competing in the women’s elite capacity if they had been through any part of the male puberty process.

Until then, athletes had been permitted to compete provided they lowered their testosterone levels.

The Court of Arbitration for Sport (Cas) have confirmed that Thomas has registered a request for arbitration “aimed at challenging certain parts of World Aquatics’ Policy on the Eligibility for the Men’s and Women’s Competition Categories”.

The 25-year-old became the first transgender athlete to win an NCAA Division I national championship in any sport when she took 500-yard freestyle gold in 2022.

She began swimming on the men’s team at the University of Pennsylvannia in 2017, before she began transitioning two years later.

The NCAA championship in 2022 remains Thomas’s last competitive meet, and she has not commented on the reports of her legal challenge

The Cas arbitration process commenced in September of last year, but no hearing date has yet been fixed.

“Ms Thomas accepts that fair competition is a legitimate sporting objective and that some regulation of transgender women in swimming is appropriate,” a Cas statement said.

“However, Ms Thomas submits that the Challenged Provisions are invalid and unlawful as they discriminate against her contrary to the Olympic Charter, the World Aquatics Constitution, and Swiss law including the European Convention on Human Rights and the Convention on the Elimination of All Forms of Discrimination against Women; and that such discrimination cannot be justified as necessary, reasonable, or proportionate to achieve a legitimate sporting objective.

“In bringing the matter before CAS, Ms Thomas seeks an order from the CAS declaring that the Challenged Provisions are unlawful, invalid, and of no force and effect.”

A number of sporting bodies have blocked transgender, male-to-female athletes from participating in elite female sport, including cycling and athletics.

In a statement issued to BBC Sport, Brent Nowicki, the executive director of World Aquatics, said: “[Our] policy on gender inclusion, adopted by World Aquatics in June of 2022, was rigorously developed on the basis of advice from leading medical and legal experts, and in careful consultation with athletes.

“World Aquatics remains confident that its gender inclusion policy represents a fair approach, and remains absolutely determined to protect women’s sport.”

World Aquatics, formerly known as Fina, shelved plans to launch an open category at its Swimming World Cup event in Berlin late last year after no athletes entered. The body said it still planned to offer the category at future events.