"After six years of marriage, and having been separated for the past year, Katharine McPhee has filed for a dissolution of marriage from Nick Cokas," her rep confirmed to People.
"The two have an amicable relationship and will remain friends. They ask that people respect their privacy during this time."
American Idol finalist Katharine had already separated from Nick when she was spotted kissing Michael, who is married to actress Mary McCormack, in October last year.
Michael and Mary, who have three young daughters together, have reportedly since rekindled their marriage.
Initially it had seemed that Katharine and Nick might also reconcile, when they were spotted walking their dogs together in March and looking very happy.
However, they ultimately decided to part ways. "It was just time," a source told People. "They'd grown apart and weren't right for each other anymore."
Katharine and Nick married in Beverly Hills in February 2008, when Katharine was 23 and he was 42.
Their age gap wasn't a problem. "When I turned 21, I thought it was a good age to explore," Katharine previously told Stuff magazine. "You're no longer a teenager, so it's OK (to date older men). I gravitated towards them, and I love them.
"They're just wiser, and they're less about themselves and more about you."