Kristen Stewart allegedly cheated on Robert Pattinson

Kristen Stewart reportedly had a fling with her 'Snow White and the Huntsman' director, Rupert Sanders.

Kristen attended the Teen Choice Awards with her 'Twilight' costar and real-life boyfriend Robert Pattinson just two days ago, however an American magazine cover alleges she strayed away from the relationship.

Us Weekly magazine has plastered a photograph of her and Rupert on its cover that would indicate the two had a romantic relationship.

Meanwhile, a source told People magazine: "Kristen is absolutely devastated. It was a mistake and a complete lapse in judgment."

Reps for neither Kristen, Rob nor Rupert have commented on or moved to clarify what went down, including the possibility of whether this is all one big misunderstanding.

A source close to Robert told E! News that he has always found fame "very difficult and isolating" and has long thought of Kristen as "the only person who he can talk to about that".

They added: "He feels like they are both on the same level and they understand one another.

"He worships her. He doesn't think there is another woman on the planet who is as beautiful and kind as her. He wants to be with her 24/7, he wants to marry her...as far as he is concerned, Kristen is his perfect match...Kristen is perfect to him. That's why he's always known that someday she would be his wife."

Rupert is married to 33-year-old model, Liberty Ross - who also starred in 'Snow White' as Queen Eleanor. They have two young children, Skyla and Tennyson, together.