Music producer Robert "Mutt" Lange is being accused of cheating on his superstar wife, Shania Twain. Sources tell PEOPLE magazine that the reason for the couple's split after 14 years of marriage was Lange's relationship with another woman.

Lange gave PEOPLE a very simple reason for the split, calling it "literally just a growing apart, that's all," he said. But sources allegedly close to the couple say infidelity is the real reason, claiming Lange had an affair with Marie-Anne Thiébaud, a secretary and manager of the couple's chateau in Switzerland.

"Mutt and Marie-Anne left their spouses for each other and are still in a relationship," says one source.

Another source says Twain is "devastated. This came out of left field . . . She loved him."

Lange is denying the alleged affair. "It's absolutely not the reason [for the separation]," he told PEOPLE. "It's not true."

Twain and Lange married in 1993. They have a 6-year-old son, Eja.