But the scandal involving the on- and off-screen couple is unlikely to affect box-office returns for the final installment in the vampire-romance juggernaut due this fall, or even harm the image of the 22-year-old actress.
"It could make her actually more alluring," said Ian Drew, a senior editor at Us Weekly magazine, which features compromising photos of Stewart and her "Snow White and the Huntsman" director Rupert Sanders in its latest issue, out Friday. "It's not like Sally Field did this, so it could actually enhance her appeal and make her even bigger."
Stewart, whom Forbes named Hollywood's highest-paid actress last month, issued an apology to People magazine Wednesday, saying she is "deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected."
"This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob," she said. "I love him, I love him, I'm so sorry."
Stewart and Pattinson, 26, have been in a relationship for several years after meeting on the set of "Twilight," in which they play lovers.
Sanders, 41, who is married and has two children, followed with his own apologetic statement to People.
"I am utterly distraught about the pain I have caused my family," he said. "My beautiful wife and heavenly children are all I have in this world, I love them with all my heart. I am praying that we can get through this together."
A spokesman for the director confirmed the statement Wednesday. Representatives for Stewart and Pattinson did not respond to requests for comment.
Fans embraced Stewart as human girl Bella Swan and Pattinson as vampire suitor Edward Cullen from the moment they were announced in 2008 as the stars of the big-screen adaptation of Stephenie Meyer's popular novels. The real-life romance that bloomed between the co-stars only made things more magical for the mostly female fan base.
"The fans are so romantically tied to this movie in both the real-life romance and the on-screen romance, so I'm sure this is hitting them pretty hard," said box-office analyst Paul Dergarabedian of Hollywood.com. "They think of Bella and Edward and Kristen and Rob like family - characters they absolutely love on screen and in real life. For a 14-year-old girl, this is probably heartbreaking. But are girls not going to see the movie because of this? Heck no."
Reports of infidelity could even draw more viewers to theaters for "The Twilight Saga: Breaking Dawn - Part 2," he said: "They might go just so they can be even more mad at Kristen."
Pattinson became an instant heartthrob with his casting as the charming, elegant Edward Cullen. He was included among People magazine's "Sexiest Men Alive" in 2008 and has won similar titles from readers of Glamour and OK magazines.
"The shocking thing is (Stewart) cheated on Robert Pattinson, who is this guy who everyone in the world wants," Drew said.
Fans might feel it when they watch the film in November.
"It sort of intrudes on their universe a little bit," he said. "This is the ill-fated romance from the screen that ended up working on screen and in real life as well. That made it more believable and more sellable, so it punctures holes in that."
Still, moviegoers might find the changed dynamic compelling.
"There's a soap opera going on off-screen, and people love to follow that," Dergarabedian said. "I think it only serves to raise awareness of the movie."