(Cover) - EN Showbiz - Kristen Stewart reportedly cheated on boyfriend Robert Pattinson with a married director.

The alleged affair apparently happened recently, sources have told People magazine.

The Twilight co-stars relationship is said to have been rocked by the startling claim.

"Kristen is absolutely devastated," one insider told the publication. "It was a mistake and a complete lapse in judgment."

"She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened.

"She never meant to hurt anyone. She's a good person who just made a bad choice."

Kristen, 22, and Robert, 26, seemed to be in good spirits just a few days ago at Comic-Con and were seen laughing and joking together.

The pair has been dating for several years after getting together soon after production started on the hit vampire franchise. (C) Cover Media