Michael Douglas, 68, and Catherine Zeta Jones, 43, married in 2000 after meeting in France two years earlier.
They have two children – Dylan, 13, and Carys, 10.
The pair have both struggled with health problems in recent years.
Michael Douglas was diagnosed with throat cancer in 2010, while Catherine Zeta Jones sought treatment for bipolar disorder in 2011 and again earlier this year.
The couple share the same birthday, 25 September, and met at the Deauville Film Festival in France in 1998.
Michael Douglas and Catherine Zeta Jones had their first child in August 2000, and married three months later in a lavish ceremony at New York’s Plaza hotel, where guests included Goldie Hawn, Christopher Reeve, Jack Nicholson and Brad Pitt.
OK! magazine paid $1.5 million for exclusive access to photographs of the day, but rival magazine Hello! spoiled the scoop by publishing pictures taken by somebody pretending to be a guest or a waiter.
A long legal battle followed in the UK, with the couple arguing that Hello! Magazine had invaded their privacy.
The case was eventually resolved in 2007, when the House Of Lords ruled Hello! had breached OK!‘s confidentiality.
Prior to the wedding, Michael Douglas and Catherine Zeta Jones reportedly signed a pre-nuptial agreement, which entitles Zeta Jones to $1.5 million for every year of their marriage in the event of the relationship coming to an end.
Michael Douglas insisted on the contract after his first wife, Diandra, was awarded $70 million in their divorce.
Speaking to the Mirror earlier this month, Michael Douglas said his biggest regret was not ending his first marriage sooner. He had married Diandra in 1977 and they separated in 1995, but did not file for divorce until 2000.
“I have nothing against her and in fact I’m very fond of my first wife, but we should have ended that marriage eight or 10 years earlier,” Michael Douglas said.
“It took me too long to realize that if you go to a marriage counsellor to resolve problems, it’s in his interest to keep the marriage going.”
In the same interview, Michael Douglas paid tribute to Catherine Zeta Jones, who had given him “this second chance to have a family”.
“The thing I’ve learned about getting older is you can’t take love for granted,” he said.
“You protect it, nurture it and it grows and after one’s initial, physical emotional aspects, it becomes deeper.”
Michael Douglas, whose films include Wall Street and Basic Instinct, recently made his post-cancer return to the big screen playing Liberace in the biopic Behind The Candelabra.
Catherine Zeta Jones, an Oscar-winner for her role in Chicago, is currently starring alongside Helen Mirren and Bruce Willis in popcorn action film Red 2.
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