Tennis great Martina Navratilova has signed a letter in support of an Idaho law that bans transgender women and girls from playing on female high school and college sports teams.

Idaho Governor Brad Little, a Republican, signed House Bill 500 in March.

Navratilova was one of 309 athletes to sign the letter in support of the law. The letter was organized by Save Women's Sports.

The letter calls on the board of governors of the National Collegiate Athletic Association (NCAA) to “reject the call to boycott Idaho for protecting the integrity of women's sports.”

Civil rights activists have called on the NCAA to move men's basketball tournament games out of Idaho in response to the law's passage.

LGBT sports blog Outsports was first to report that Navratilova, who is openly gay, had signed the letter. Only the names of eight athletes had been previously released to the public.

Save Women's Sports criticized the publication of Navratilova's involvement with the campaign. “It is disgraceful that Outsports and other like-minded extremist activist groups are bullying and harassing women simply for standing up for fairness in the sports they love.”

Navratilova has previously voiced her opposition to transgender women participating in female sports teams.

(Related: Martina Navratilova apologizes for calling transgender athletes “cheats.”)