TIM MCGRAW and record label mogul Mike Curb made beautiful music together for nearly 20 years, but the harmony turned nastily off-key after McGraw accused Curb of treating him like a servant!

Curb, a popular music group leader in the 1960s who went on to become lieutenant governor of California, signed a then-unknown McGraw to his newly launched Curb Records in 1993. McGraw became the label’s most popular artist and biggest moneymaker.

But McGraw said he felt like he was working in “indentured servitude” to Curb, 69, sources say, when the label continued to release Tim McGraw greatest hits albums in order to extend the star’s contract with the label indefinitely.

“Curb out out five greatest hits albums in eight years when Tim wanted to make new albums instead,” a source told The ENQUIRER. “He figured fans were getting ripped off buying the same songs over and over when they should get new material.”

Fed up, McGraw, 47, hit the recording studio in 2010 to work on his “Emotional Traffic” album – and the two wound up filing lawsuits against each other.

With his wife Faith Hill standing by, McGraw won his release from Curb Records in November 2011. And while he now records for Big Machine Records, legal action in the case continues, and a source says the animosity between Curb and McGraw still runs high.

“Mike feels like Tim acted like a terrible ingrate, but Tim thinks he was being taken advantage of,” said the source. “It’s become one of the nastiest battles ever in country music."