AB Hernandez (center) poses as she shares the first-place spot on the podium with Jillene Wetteland (fourth from left) and Lelani Laruelle (fourth from right) during a medal ceremony for the high jump at the California high school track and field championships in Clovis, California, Saturday. AP-Yonhap
AB Hernandez (center) poses as she shares the first-place spot on the podium with Jillene Wetteland (fourth from left) and Lelani Laruelle (fourth from right) during a medal ceremony for the high jump at the California high school track and field championships in Clovis, California, Saturday. AP-Yonhap

WASHINGTON (AFP) — US President Donald Trump has vowed to impose "large scale" fines against California after a transgender high school athlete's victory at state track and field championships last weekend.

AB Hernandez, 16, won two gold medals and a silver at the meet in Clovis, amid an outcry over her participation.

Hernandez shared her place on the medal podium with rivals under modified rules applied specifically to her events designed to ensure that "biological female student-athletes" were not deprived of winning medals.

Trump, who had protested against Hernandez's participation last week, said in a Truth Social post late Monday that California authorities would be fined as a result.

"A Biological Male competed in California Girls State Finals, WINNING BIG, despite the fact that they were warned by me not to do so," Trump wrote.

"As Gov. Gavin Newscum fully understands, large scale fines will be imposed!!!" added Trump, using the disparaging moniker he reserves for California's Gov. Gavin Newsom.

The US Department of Justice last week announced it had launched an investigation into whether California had violated Title IX, the law that prohibits sex discrimination in educational programs that receive federal funding.

"The investigation is to determine whether California, its senior legal, educational, and athletic organizations, and the school district are engaging in a pattern or practice of discrimination on the basis of sex," the DOJ said in a statement.

On Monday, it followed up that announcement with a letter to the California Interscholastic Federation warning the body that the rules that allowed Hernandez to compete violated federal law.

The Justice Department ordered the CIF to confirm in writing by June 9 that its bylaw would not be implemented.

"Scientific evidence shows that upsetting the historical status quo and forcing girls to compete against males would deprive them of athletic opportunities and benefits because of their sex," the letter from Assistant US Attorney General Harmeet Dhillon read.

"Therefore, you cannot implement a policy allowing males to compete alongside girls, because such a policy would deprive girls of athletic opportunities and benefits based solely on their biological sex, in violation of the Equal Protection Clause."