There are rumors of backstage battles among the superstar coaches on NBC's The Voice.

While Miley Cyrus is in the thick of her first season as a coach on The Voice, she is not returning for the upcoming spring edition, season 12. Gwen Stefani, who coached in seasons 7 and 9, is set to return for season 12 alongside Adam Levine, Alicia Keys and Blake Shelton.

Now there are rumors of Shelton specifically requesting Stefani as a coach alongside him in season 12.

"Blake made it clear to the producers that he wanted Gwen by his side for the upcoming season. It almost seemed like if she did not return then neither would he!" a source told RadarOnline.com recently.

The supposed insider added that Cyrus hasn't necessarily been a huge buzz generator as producers may have hoped for.

"Miley did bring in a younger audience but she did not generate the kind of buzz that Gwen did by being on the show with Blake," they said. "Miley did not get along with the cast and the crew as well, either."

Meanwhile, Cyrus is set to return next fall for the 13th season of The Voice. There's no word on whether Stefani will also be coaching in season 13.

"They offered Miley the gig for Season 13 because they know it would keep her happy, and that she would not create any drama over being replaced by Gwen," the source claimed.

Of course, all sources should be treated with a grain of salt, as NBC, The Voice producers and the various celebrity coaches have not confirmed the validity of these claims.

In the mean time, there is only one more knockout performance show left on this season of The Voice, meaning the live shows are ahead. The Voice airs Mondays and Tuesdays at 8/7c on NBC.