According to FemaleFirst, the Notting Hill star warned the actress that he would attack Matthew Rhys if he were to find out that Miller was cheating on him.
Last week, Ifans was devastated by the breakup, but now it seems as though he is out to get revenge. For instance, he threatened Sienna Miller saying that he would publicly humiliate her during her upcoming film premiere for The Edge of Love, which takes place on June 19 at the Edinburgh Film Festival.
A source told The Sun, "His hurt has now turned to anger and he has said he will sort Matthew out when he sees him. His last message to Sienna said he will ruin her big night at the premiere. She thinks he is serious."
Miller broke up with Ifans because of his jealousy of Miller’s co-star and ex-boyfriend, Matthew Rhys, in the film, The Edge of Love.