Uh oh as it appears Miley Cyrus is super jelly of her fiance Liam Hemsworth's newest costar Emma Roberts!

The latest issue of the National Enquirer is reporting that Miley, 19, is worried Emma, 21, will steal Liam away from her.

"Miley's become super-protective since she and Liam got engaged," reveals a source. "She's worried that while he's off making movies with hot women, he'll be tempted to cheat."

Not helping matters is the fact that Emma is currently not in a serious relationship (though a recent report says she is dating a new guy). "And it didn't help that Emma showed up on set declaring that she is single and ready to mingle," adds the source.

Miley also reportedly blew up after she learned that Liam, 22, would be hanging out with Emma and another co-star Nikki Reed, during off-work hours.

"Miley doesn't want Liam socializing with a loose cannon like Emma," the insider states. "She thinks the girl is bad news. Not only did she tell Liam it had to stop, she even confronted Emma, reminding her that Liam is off the market."

Emma is however not concerned with Miley's insecurities. "She couldn't care less what Miley thinks," adds the source. "She makes fun of Miley behind her back." Ouch! Liam is also said to be "definitely enjoying" the attention Emma is giving him.

TELL US - THOUGHTS? IS EMMA A THREAT TO MILEY?