A friend told People, “The stress has taken a toll on their marriage.”
The couple hasn’t been photographed together since April 22. People reports that they made the decision to separate after the Cannes Film Festival in May.
So far, the couple has not filed for divorce or a legal separation.
Douglas, 68, and Zeta-Jones, 43, have two children together, Dylan, 13, and Carys, 10. An insider said, "They want the best for their kids, no matter what happens."
The new issue of People hits newsstands on August 30.