A coalition of advocacy groups are preparing to mount a legal challenge against a Washington state ballot initiative that bans transgender girls’ participation in girls’ school sports, which voters approved in last week’s midterm election and will go into effect next month.

The challenge is poised to set off a protracted legal battle over the rights of trans youth in Washington, which has functioned as a safe haven for thousands of trans people fleeing states that have enacted anti-trans laws in recent years.

If enforced, the ballot measure, Initiative 638, would be “the most extreme” trans sports ban of its kind, according to its opponents. The initiative’s official text says it would require all girls participating in school sports to “verify biological sex”, which could include confirming their “reproductive anatomy”.