AceShowbiz -

One of Hollywood's most celebrated couples, Michael Douglas and Catherine Zeta-Jones, is on the verge of calling it quits after 13 years of marriage. Representatives for both stars confirmed, "Catherine and Michael are taking some time apart to evaluate and work on their marriage."

The pair haven't been pictured attending a public event together since April when they posed on the red carpet at the 40th Annual Chaplin Award Gala at Avery Fisher Hall in New York. Douglas was recently spotted on a yacht off the coast of Sardinia, while Zeta-Jones stayed at their home in New York.

There is no official word as to why "they're taking a break" from their relationship. A friend reveals to PEOPLE that the stress between the "Haywire" actor's throat cancer battle and the "Rock of Ages" actress' struggles with bipolar II disorder "has taken a toll on their marriage."

An interview Douglas gave to the Guardian earlier this year about his cancer being caused by HPV which he said he got from oral sex reportedly further strained their relationship. Also, there were Cameron's legal woes. The actor's son from his first marriage to Diandra Luker is currently serving jail time for drug problem.

Rumor suggests Douglas, 68, and Zeta-Jones, 43, decided to take separate time from each other shortly after he returned from the Cannes Film Festival in May to promote his movie "Behind the Candelabra". She herself just returned from her second treatment for bipolar disorder back then.

Douglas and Zeta-Jones married in 2000 in a lavish ceremony at Manhattan's Plaza Hotel. They have two children together, Dylan, 13, and Carys, 10. Neither has filed for divorce or moved toward a legal separation. A family friend says the couple hopes to work on their differences. "They want the best for their kids, no matter what happens," the source says.