The allegations were made by American magazine Us Weekly, which claims to have photographic evidence of the fling, which they say happened in July.
According to sources, Kristen Stewart was snapped kissing Rupert Sanders in LA on July 17th, which led to the magazine's explosive claims.
America's leading celebrity website, People.com, have also repeated the claims, with their sources revealing the kiss was 'a complete lapse of judgement' by Kristen Stewart, who they say is 'devastated' at the turn of events.
'Kristen is absolutely devastated,' one source told the site. '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.'
The insider continued: 'She never meant to hurt anyone. She's a good person who just made a bad choice.'
If the reports are true, Kristen Stewart's indiscretion comes less than a week after she hit Comic-Con with british boyfriend Robert Pattinson, were they were spied snuggling up for the cameras, and flirting during a press conference.
Last night, Rupert Sanders model wife Liberty Ross, with whom his has two children, posted a series of cryptic Tweets, including a quote from Marilyn Monroe, which read: 'Sometimes good things fall apart so better things can fall together.'
But today, the model's Twitter account has been deleted.
Yesterday, Black Book published an interview with Robert Pattinson, in which he hinted marriage to Kristen Stewart might just be on the cards, admitting the actress 'has always done something to me that others haven’t.'
A rep for Rupert Sanders has yet to be reached, while Kristen Stewart's rep has refused to comment on the affair claims.
What do you think of today's wicked whispers? Is is true Kristen Stewart cheated on Robert Pattinson with Snow White and the Huntsman director Rupert Sanders? Or is the story total rubbish? Let us know in the comments box below…