Kristen Stewart Cheats on Rob Pattinson with Rupert Sanders

Kristen Stewart allegedly cheated on Robert Pattinson on Tuesday (July 17) with another Brit, her Snow White and the Huntsman director, Rupert Sanders.

Us Weekly reports they had a “marathon makeout session.”

“Kristen is absolutely devastated,” a source tells People. “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.”

Rupert, 41, is married to British Vogue model Liberty Ross, 33, and they have two children, ages 7 and 5.

Rob, 26, and Kristen, 22, were last spotted together at the 2012 Teen Choice Awards. They will soon be promoting their upcoming Twilight Saga: Breaking Dawn — Part 2, out November 16.