Boxer Christy Martin has defended Sydney Sweeney's performance in her biopic Christy after Ruby Rose claimed the actress ruined the movie.

The former professional boxer, whom Sweeney portrays in the new biopic, took to Instagram on Wednesday to come to the Euphoria star's defence after seeing others "attack (her) friend" online.

"I've always been a fighter. My life reflected that in every way and now I fight for others. The past few days I have seen some people attack my friend Sydney Sweeney," she wrote alongside a picture of her with Sweeney on the Christy set. "Syd not only worked her a*s off for this film, she worked her a*s off for me. For my story. For so many others suffering in silence. So I want to be clear about who Syd is. She is my friend and ally!"

While Martin didn't name anybody specifically, her post came the day after Orange Is the New Black actress Rose criticised Sweeney's involvement in the boxer's life story.

Rose claimed she was once attached to play a fictional character based on Martin's high school girlfriend Sherry, and the original script was "incredible" and "life changing".

"Everyone had experience with the core material. Most of us were actually gay," she wrote on Threads, presumably referring to the cast line-up before the addition of Sweeney. "It's part of why I stayed in acting. Losing roles happens all the time.

"For her PR to talk about it flopping and saying SS did it for the 'people'. None of 'the people' want to see someone who hates them, parading around pretending to be us. You're a cretin and you ruined the film. Period. Christy deserved better."

Rose's comments seemingly refer to reports suggesting the Anyone But You star is a registered Republican voter. She has yet to address her political affiliation.

Christy opened in U.S. cinemas on 7 November and grossed only $1.3 million (£988,000), making for one of the worst U.S. debuts ever for a movie opening in more than 2,000 screens.

Addressing its disappointing performance, Sweeney wrote on Instagram, "If christy gave even one woman the courage to take her first step toward safety, then we will have succeeded. so yes I'm proud. why? because we don't always just make art for numbers, we make it for impact. and christy has been the most impactful project of my life."