Jennifer Lopez and Marc Anthony are done playing friendly exes. Things between the former couple are getting "seriously ugly," according to Us Weekly. Multiple insiders tell the magazine that Jennifer's hot romance with 24-year-old beau Casper Smart is what turned things sour between the American Idol judge and her ex-husband.

According to sources, Anthony isn't happy that Smart is getting along so well with Max and Emme, his three-year-old twins with Lopez. Paparazzi pics of his ex-wife and children vacationing with Smart in Hawaii over Thanksgiving made him "hit the roof," one source says. "It caused a huge blowup."

His retaliation? Going "back on his word" over the custody agreement between him and Lopez, in which Jennifer gets primary custody of their twins. "He wants to make Jennifer suffer because she's having fun with someone new, and he knows this will hurt her on the deepest level."

But Lopez is ready to fight back, says another insider. She's "ready to smash him if he tries anything…It's getting vicious."

Apart from their children, the couple also share custody of numerous business ventures, including clothing lines with Kohl's and the Latin American talent search show Q'Viva: The Chosen, which they host together but have been filming separately.

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