Elizabeth Warren has spoken out against an Arizona bill that would stop female transgender athletes from competing in female sports.

The bill, which is called the "Save Women's Sports Act" has been introduced by Rep. Nancy Barto, R-Phoenix. 

The bill would require sports to be designated as co-educational, male or female according to the legal definition of "biological sex". It would apply to all schools and colleges in the state. 

Warren, who is running to become the next Democratic presidential candidate wrote on Twitter: "Trans athletes are not a threat. We need to protect trans kids—and all LGBTQ+ kids—and ensure they feel safe and welcomed at school. I urge the Arizona legislature to reject this cruel bill."

In a statement to The Arizona Republic about the bill, Barto said: "Female student-athletes should not be forced to compete in a sport against biological males, who possess inherent physiological advantages. When this is allowed, it discourages female participation in athletics and, worse, it can result in women and girls being denied crucial educational and financial opportunities."

LGBTQ+ rights groups are against the measures and it isn't the first time Warren has spoken about protecting the rights of transgender students. 

In January, Warren tweeted: "Trans youth are more likely to feel unsafe at school and to experience mental health problems. They need and deserve to be treated with dignity and respect, not to be attacked by their state legislators. As president, I’ll fight to ensure they have every opportunity to thrive."

Georgia, Missouri, New Hampshire, Tennessee, Washington, and West Virginia also have had similar bills introduced

Warren is a vocal campaigner for both women's and LGBTQ+ rights and has been endorsed by the likes of Megan Rapinoe, who wrote on Twitter: "I truly believe the best things in life are a result of being bold and being real. I’m proud to endorse Elizabeth Warren today, for being bold, for being real, for listening to ALL of us, and for being prepared to navigate the unique challenges we face today as a country."