kristen-stewart-rupert-sanders.jpgHold on to your seats, “Twilight” fans. According to a report from People magazine, Kristen Stewart cheated on long-time boyfriend Robert Pattinson with her “Snow White and the Huntsman” director Rupert Sanders.

The bombshell came in an anonymously-sourced item posted Tuesday (July 24) to the magazine’s website.

“Kristen is absolutely devastated,” a source tells People. “It was a mistake and a complete lapse in judgment.”

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“She wasn’t having an affair with Rupert,” adds the source. “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.”

Neither Sanders nor Stewart have commented on the report. Obviously.

Stewart and Pattinson recently appeared together at Comic-Con to hype the final installment of the “Twilight” saga, “Breaking Dawn – Part 2.”

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UPDATE: Us Weekly posted a copy of their new cover, which includes a purported picture of Stewart with Sanders and details on how the alleged liaison unfolded. Us claims the pair was caught together — at least on camera — on July 17 in a “series of steamy rendezvous.”

Us also adds the detail that Sanders is engaged to model Liberty Ross, with whom he has two young children.

kristen stewart rupert sanders us cover Kristen Stewart cheated on Robert Pattinson with 'Snow White' director Rupert Sanders   report