Robert Pattinson has been left "heartbroken and humiliated" after girlfriend Kristen Stewart cheated on him.
The heartthrob was stunned when he discovered his partner of four years had been caught kissing director Rupert Sanders - who she worked with on Snow White and the Huntsman - and sources said he was struggling to cope with the news.
One source - who saw Pattinson and Stewart fighting backstage at the Teen Choice Awards in Los Angeles on Monday - said: "Rob and Kristen were having a long and intense conversation backstage, and he looked miserable, heartbroken and humiliated. She was pleading."
Another source added to the New York Post newspaper: "Rob is deeply in love with Kristen and is very jealous when other men hit on her. This will crush him."
After pictures emerged of Stewart and Sanders kissing, the 22-year-old actress issued a statement saying how sorry she was.
"I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected. This momentary indiscretion has jeopardised the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry," Stewart said.
British director Sanders is married to model and actress Liberty Ross, 33, with whom he has two children. In a twist befitting of a good Hollywood drama, Ross portrayed the mother to Stewart's Snow White in the Huntsman tale.
The 41-year-old also issued a statement today saying: "I am utterly distraught about the pain I have caused my family. My beautiful wife and heavenly children are all I have in this world. I love them with all my heart. I am praying that we can get through this together."
A fling between a Hollywood star and her director is nothing new in showbiz, but this one gained widespread attention because of the high-profile people involved.
Stewart and Pattinson portray the vampire-human couple of the Twilight series of blockbuster romance movies who are deeply in love in the films.
The pair's on-screen romance transferred to their real lives, and they confirmed they were dating in 2010. But they have tried to remain discreet in public and do not often discuss their relationship.
Given their past, Stewart's romantic liaisons with director Sanders, which was reported with photos on celebrity news outlets including People and Us Weekly, surprised celebrity watchers and put the trio in a publicly awkward situation.
Most recently, Pattinson and Stewart were together promoting the upcoming final Twilight film, Breaking Dawn - Part 2, at San Diego's Comic-Con earlier this month.
Universal Pictures announced in June that they were moving forward with a sequel to Snow White and the Huntsman, but did not confirm whether Sanders would helm the film.
Stewart is expected to reprise her role, but is not confirmed.