Demi Lovato vs. Britney Spears: It's War!

They're former Disney darlings turned pop stars who've battled personal demons in the public eye, and now they're judging The X Factor.

With everything they have in common, you'd think Britney Spears, 30, and Demi Lovato, 19, would become fast friends. But in fact, the two singers can't stand each other — and their fiery feud is making working on the X Factor set unbearable!

"Tensions are at the boiling point," a show insider tells Star. "There is a bitter rivalry between them, and it's already pretty much exploded — which is crazy considering how early into filming it is."

"Demi thinks Britney is a twit, and Britney thinks Demi is beneath her," another source says. "They avoid each other as much as possible when the cameras aren't rolling."