Trans swimmer Lia Thomas has mounted a legal challenge against World Aquatics

Transgender swimmer Lia Thomas has been quietly mounting a legal battle against World Aquatics to overturn the swimming governing body’s effective ban on most trans women competing in the highest levels of the sport, a lawyer representing Thomas confirmed to NBC News on Friday.

Carlos Sayao, a partner at top Canadian law firm Tyr, said Thomas is asking the Court of Arbitration for Sport in Switzerland to overturn the new World Aquatics rules, issued in June 2022, that prohibit trans women from competing in women’s swimming events unless they transitioned before age 12.

The U.K.'s Telegraph was the first to report on Thomas’ behind-closed-doors legal challenge in an article published Thursday evening. Details of Thomas’ challenge, which The Telegraph reported began in September, were not made public previously because cases brought before the Court of Arbitration for Sport are meant to be kept confidential by all parties involved.

The new rules, which would effectively bar trans women from competing in women’s swimming events at the Olympics, came several months after Thomas, then a student at the University of Pennsylvania, made history by becoming the first openly transgender woman to win an NCAA swimming championship. And in May 2022, Thomas told ABC News’ “Good Morning America” that it’s been a lifelong goal of hers to compete in the Olympics.

Thomas made global headlines for her NCAA win and became the face — and often conservative media’s punching bag — of the worldwide debate over whether trans women should compete in women’s sports.

Sayao confirmed his comments to The Telegraph regarding the rules imposed by World Aquatics, which he called “discriminatory” and said caused “profound harm to trans women.”

“Trans women are particularly vulnerable in society and they suffer from higher rates of violence, abuse and harassment than cis women,” he told the British newspaper.

Sayao declined to comment further. World Aquatics did not immediately return requests for comment.

The Court of Arbitration for Sport, or CAS, confirmed the legal challenge in a statement on Friday after an original version of this article was published.

"Ms Thomas accepts that fair competition is a legitimate sporting objective and that some regulation of transgender women in swimming is appropriate," the statement reads. "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."

The international sporting court also noted that its cases are normally dealt with in private but that the legal parties involved in Thomas' case "agreed that general information concerning the procedure itself be communicated by the CAS Court Office."

A hearing for Thomas's legal challenge has not yet been set, according to the statement.

This article was originally published on NBCNews.com