Mixed martial arts champion Ronda Rousey has refused to fight against trangender athlete Fallon Fox.

Fox, who became the first openly transgender athlete in the history of MMA when she came out last year, has won 5 of her past 6 matches.

However, Women’s Bantamweight Champion Ronda Rousey says she won’t take on Fox, because she doesn’t think there is an “undo button” on gender.

Rousey said: “In Fallon Fox’s case, I think she does have an unfair advantage.

“It’s a case by case scenario thing. I’ve tried to research it a lot.

“I feel like if you go through puberty as a man it’s something that you can’t really reverse. You can’t just reverse that, there’s no undo button on that.

“That’s unfortunately her scenario.”

Despite the fighter’s claims, researchers have been quick to point out that a trans woman would actually be at a significant disadvantage in terms of bone strength and muscle density, as a result of hormone treatments.

Fox posted on Facebook: “I really wish Ronda would stop with the ridiculous bone structure arguments.

“That was so last year. Second, she should tell her boss to put me in that octagon over there at UFC.

“I’m quite sure that there are quite a few female MMA fighters who have guts to fight another skilled woman without peeing their panties.”