Reached by PEOPLE after the separation was announced in a statement, Lange called the split "an unfortunate thing." He added, "It's literally just a growing apart, that's all."

But sources close to the situation say a third party was involved: Marie Ann Thiebaud, a longtime secretary and manager of the couple's chateau in Switzerland. "Mutt and Marie Ann left their spouses for each other and are still in a relationship," says one source.

Lange denies the claims. "It's absolutely not the reason [for the separation]," the famously reclusive producer told PEOPLE when asked about the alleged affair. He also denied that he was in a romantic relationship with Thiebaud: "I'm not, no," he said. "It's not true." (Thiebaud could not be reached for comment.)

Twain, 42, and Lange, 59, married in 1993. They have a 6-year-old son, Eja (pronounced "Asia") D'Angelo.

An insider says the "You're Still the One" singer is "devastated. This came out of left field. . . She loved him." Another source close to Twain tells PEOPLE the Canadian-born star is coping with the help of "a very close-knit group of family and friends."