ET has the latest...

Less than a week after they announced they were separating, sources are telling People the reason behind singer SHANIA TWAIN and her producer husband ROBERT "MUTT" LANGE's sudden split.

A source tells the mag that Lange was having an affair with MARIE ANN THIEBAUD, an employee at the couple's Swiss chateau.

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

But when People contacted Lange, he denied the claim. "It's absolutely not the reason [for the separation]." Adding that he was not in romantic relationship with Thiebaud, he said, "I'm not, no. It's not true."