Parents of student athletes in Washington state have sued to block a ballot initiative banning transgender girls from women’s sports teams, calling the measure discriminatory and warning it would lead to “invasive” genital examinations for female athletes.

Last week, voters in the solidly Democratic state passed Initiative 638, a major setback for LGBTQ+ rights amid the “blue wave” of the US midterm elections. Nebraska, Colorado and Arizona adopted similar anti-trans sports bans, though advocates say Washington state’s policy is particularly extreme due to its medical sex verification requirement for all girls who want to play team sports.

The lawsuit was filed in state court on Monday by parents of trans and cisgender girls; TransFamilies, a non-profit Washington advocacy group; and the Washington Education Association, the teachers’ union. They are represented by the American Civil Liberties Union of Washington; Legal Voice, a group that fights gender discrimination; and Pacifica Law Group, a local firm.

Washington’s measure mandates that girls provide documentation from medical providers about their sex before they join women’s teams, either through hormone or genetic tests or examinations of their “reproductive anatomy”. The policy does not apply to boys.

Doctors, teachers and parents have raised alarms about the potential of forcing girls to undergo genital exams, saying the enforcement mechanism was a severe privacy violation and would discourage all girls from joining sports. Proponents argue that the ban, funded by Brian Heywood, a conservative hedge fund manager, protects “fairness” in girls’ sports.

The law applies to students in elementary, middle and high school. School officials said earlier this year that they were aware of only about 10 trans sports players statewide, a tiny fraction of the state’s 200,000 student athletes.

The suit argues that the measure violates Washington’s constitution, including by infringing on students’ privacy and violating equal rights protections by only applying to girls. The ban also violates an anti-discrimination law that protects trans people, the suit says.

Tiffani Lennon, executive director of Legal Voice, noted that Washington state has for 20 years allowed trans youth to play on teams that match their gender: “This is a fabricated issue being manufactured to create alarm. They’re intending to send a message to trans youth that we don’t want you visible, and they are willing to put in place really extreme measures to ensure that it terrifies enough people to go run and hide.”

The plaintiffs include a parent of a 17-year-old trans girl who participates in team sports at her high school and who fears being outed and kicked out of athletics under the measure. Parents of two cisgender female athletes also joined the suit, saying they oppose the “intrusive” sex verification requirements.

“If you really want fairness in sports, why would you subject girls to sex verification? Why would you dissuade girls from participating in sports?” said Lennon. “It’s clearly discriminatory.”

On Monday, the Seattle public schools board of directors also voted in favor of allowing the district to take legal action against the ban, which is due to go into effect on 3 December.