Caitlyn Jenner has strongly criticised the Trump administration's decision to overturn federal protections for transgender school students, describing the move as a "disaster".

In a video message released via social media, the transgender Olympian and reality TV star reserved her sharpest words for attorney general Jeff Sessions, who has opposed the expansion of gay, lesbian and transgender rights.

“I have a message for the trans kids of America,” Jenner says in the Twitter video. “You’re winning. I know it doesn’t feel like it today and every day, but you’re winning.”

“Now I have a message for the bullies,” she continues. “You’re sick. And because you’re weak you pick on kids, you pick on women or anyone else you think is vulnerable. Apparently, even becoming the attorney general isn’t enough to cure some people of their insecurities.”

On Wednesday the Trump administration overturned a directive that instructed schools to allow transgender pupils them to use bathrooms and locker rooms matching their gender identities.