Ruth Wilson surprised fans in 2018 when she suddenly left her role on The Affair with no clear explanation.

Wilson, who won a Golden Globe for her performance on the show, announced she wouldn't be reprising her role as Alison for the series' fifth season.

At the time, she told CBS This Morning that, "she did want to leave but I'm not allowed to talk about why." Also telling The New York Times that "there's a much bigger story."

Now, a bombshell investigation by The Hollywood Reporter has revealed that Wilson's departure was influenced by a "hostile work environment", and "ongoing frustrations with the nudity required of her."

Ruth Wilson. (Getty)

The issues appeared to involve The Affair's showrunner, Sarah Treem, and one of the directors, Jeffrey Reiner.

"There was a culture problem at the show from the very beginning and a tone-deafness from Sarah Treem about recognising the position she was putting actors in," a source told the outlet. "Over and over again, I witnessed Sarah Treem try to cajole actors to get naked even if they were uncomfortable or not contractually obligated to."

While there was an understanding that nudity would be involved in filming the show, Wilson allegedly objected to gratuitous scenes, "where there seemed to be no clear creative rationale for the nudity other than for it to be 'titillating.'"

Dominic West, Ruth Wilson, Matt Blank, Sarah Treem, Josh Stamberg, Maura Tierney, Joshua Jackson and Julia Goldani Telles from The Affair. (Getty)

Treem has since hit back and denied the allegations.

"I would never say those things to an actor. That's not who I am. I am not a manipulative person, and I've always been a feminist," she said.

"I have devoted my entire professional life to writing about and speaking to women's issues, women's causes, women's empowerment and creating strong, complex roles for women in theatre and in Hollywood, on and off screen. It's what I think about, what I care about, it's what drives my life and work.

"The reason I even created The Affair was to illuminate how the female experience of moving through the world is so different from the male one, it's like speaking a second language," she explained. "The idea that I would ever cultivate an unsafe environment or harass a woman on one of my shows is utterly ridiculous and lacks a grounding in reality."

Ruth Wilson won a Golden Globe for her performance on The Affair. (Getty)

Wilson's choice to leave was allegedly also due to a run-in that occurred between Jeffrey Reiner and Lena Dunham in New York.

Reiner allegedly asked Dunham to have dinner with Wilson and encourage her to "show her tits or at least some vag," and then proceeded to "critique and crudely evaluate the bodies of all the women on his show."

Reiner also allegedly flashed Dunham a photo of a "mutual friend with a c--- next to her face," which sources believe was Maura Tierney, who was another actress on the show.

This incident ultimately led to Wilson and Tierney expressing "discomfort about continuing to work with Reiner," The Hollywood Reporter claimed.