Shia LaBeouf's relationship with Karolyn Pho is reportedly extremely tense at the moment.

The actor recently revealed that he might have real sex while filming provocative director Lars von Trier's upcoming feature Nymphomaniac.

Fashion designer Karolyn is not at all happy with the possibility of her boyfriend fornicating with one of his co-stars.

"They've been on the rocks ever since Shia agreed to star in Nymphomaniac," a source told Radar Online.

"Karolyn gets that Shia likes to go method and appreciates his artistic integrity. However, she thinks performing a real sex scene in a movie is going too far.

"It's difficult for her to understand why it can't be simulated just like in other films."

Karolyn hopes that other arrangements can be made.

"She wants him to do the film, because Shia was desperate to work with Lars he's such a hero of his. And she doesn't mind if he strips off completely she had no qualms about the Sigur Ros music video," the source explained.

"But Karolyn doesn't want him having sex with somebody else simple. She's begged Shia to ask Lars if it's possible to make the scenes appear real, rather than shoot them real."