JUSTIN, JUSTIN, JUSTIN!

Justin Timberlake and Jessica Biel's notoriously on-and-off relationship is once again on the rocks as the singer cozies up to ex Cameron Diaz while shooting the romantic comedy Bad Teacher, reports the new Us Weekly (on newsstands now).

Multiple sources tell Us Weekly that Diaz, 37, and Timberlake, 29, flirt openly on the Valencia, Calif., set.

"He tortured Jess by taking this role, and they are reaching a breaking point," a pal tells Us Weekly. "The truth is, I don't know if they can survive this."

While shooting, it's like "old times" for Timberlake and Diaz, who dated for four years before he began romancing Biel, 28, in 2007.

There's "lots of laughing, making fun of each other, kidding around off camera," the source says. "She's sassy with him -- and slick, prancing around in tight clothes."

Biel and Timberlake have been having "huge arguments" on the phone about it, says one source. The more Biel gets "territorial and upset," another adds, Timberlake gets turned off and turns more toward Diaz, with whom he has been having "a ball."

And though Timberlake's mom has told Us that Biel is "one of us," a source close to Timberlake tells Us Weekly he "won't marry" her because "he doesn't respect her anymore." In fact, the source says, "when she's not around, he'll make fun of her to his friends."
SOURCE