Robert Pattinson was furious when he walked in and discovered girlfriend Kristen Stewart in bed with her "On The Road" co-star, Viggo Mortensen.
A source said "He lost it completely. Took his gloves off and slapped Mortensen around the face with them and therefore, in the rules of a British gentleman, challenged him to a duel."
Mortensen is said to have been bemused at first by Pattinson actions, but once he realised Pattinson was serious, arranged a time and place with him. Stakes and fangs are said to be the weapons of choice, with Pattinson having called aupon "The Pack" to be his seconds.
The source said, "Robert was so angry. He already knew Kristen had slept with pal, Tom Sturridge, but he had no idea she was also sleeping with Mortensen as well."
Ms Stewart was not available for comment and said to be re-filming her three in a bed scene with Mortensen today.