Caitlyn Jenner has slammed Donald Trump's decision to revoke federal protections for transgender school students, saying it's "a disaster".

On Wednesday, the US President overturned landmark guidance by Barack Obama which instructed public schools to allow transgender students to use the bathrooms and locker rooms according to their gender identity. 

Now Jenner, who is a Republican and had been a vocal supporter of Trump, has spoken out against the Trump's administrations decision, describing them as "bullies" in a video shared on her Twitter page.

"I have a message for President Trump, from one Republican to another," said Jenner. "This is a disaster. And you can still fix it. You made a promise to protect the LGBTQ community. Call me."

The reality TV star opened the video with an empowering message for the transgender children of American, saying: "I have a message for the trans kids of America: you're winning. I know it doesn't feel like it today or every day, but you're winning.

"Very soon, we will win full freedom nationwide and it's going to be with bipartisan support. You can help by checking out the National Center for Transgender Equality and letting Washington hear you loud and clear."

Caitlyn Jenner speaks out against Trump's transgender rights rollback

Last month, Jenner, who detailed her transition in a Vanity Fair cover story in July 2015, offered her help to fellow Republicans in understanding LGBTQ issues.

Jenner has been an open advocate for Trump in the past.

"Trump seems to be very much for women," she said in an interview with Stat magazine in June 2016. “He seems very much behind the L.G.B.T. community because of what happened in North Carolina with the bathroom issue. He backed the L.G.B.T. community.”