Robert Pattison has packed his bags and moved out of the Los Angeles home he shared with the actress, according to People magazine.
It comes less than 24 hours after Stewart's public apology for having a fling with her Snow White and the Huntsman director Rupert Sanders.
"I'm not sure they'll be able to recover from this," a source told People of the couple, who met making the Twilight trilogy.
The source said Pattison "is heartbroken and angry."
The Daily Mail reported that while Stewart and Sanders have issued grovelling apologies for their infidelity, Pattison has remained silent, but his actions today speak volumes.
Kristen and Rupert were photographed in a passionate embrace on July 17, and the images published in the media two days ago.
The actor is reportedly not in contact with Stewart.
In a previous interview, he made his feelings regarding infidelity clear, telling Vanity Fair in April: 'There's a thing I've never got: That is, why do people cheat?'
He explained that while he "can understand the impulse" to cheat, he doesn't comprehend "how you can keep two relationships going at the same time for long."
"If I choose to be with someone, it's because I really want it. When I have a relationship, I'm 100 percent into it."
Kristen said in her apology statement: "I love [Rob], I’m so sorry."