Kristen Stewart cheated on Robert Pattinson with the married director of the 'Snow White and the Huntsman' movie Rupert Sanders, according to People magazine. Stewart and Pattinson's relationship has been rocked by the revelation that the actress had a fling with the filmmaker, though the validity of the story is being questioned.

According to sources, "Kristen is absolutely devastated" that the news has got out, adding, "It was a mistake and a complete lapse in judgment". The alleged hook-up with Sanders - who directed commercials before making his movie debut with Snow White - occurred recently, according to another insider who added, "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". A representative for Sanders - who is married with two children - is yet to comment on the allegations, while a spokesperson for Stewart had no comment. Stewart, 22, and Pattinson, 26, were all smiles during a recently appearance at Comic-Con, joking about their kissing scenes in the Twilight movies. The pair met on the first vampire film and have been largely inseparable since, despite keeping their relationship intensely private from the media. Sanders is married to the model Liberty Ross, the sister of Oscar winning composer Atticus Ross (who worked with Trent Reznor on the scores for 'The Social Network' and 'The Girl with the Dragon Tattoo'.)

US Weekly magazine provide "photographic evidence" of Kristen in a romantic clinch with the director, though according to the UK's Daily Telegraph, Twilight fans are claiming the actress' face has been digitally imposed onto Sanders' wife's body, pointing out that her shoulder is extremely bony, unlike Kristen's.