IN a hush-hush trial separation, Hollywood power couple MICHAEL DOUGLAS and CATHERINE ZETA-JONES secretly split last summer, The ENQUIRER has learned exclusively.

The blockbuster breakup came last summer after their 12-year marriage buckled under the pressure of deal­ing with Michael’s cancer battle, his druggie son Cameron’s incarcera­tion and Catherine’s struggle with bipolar II disorder, sources say.

At the peak of their problems, Mi­chael, 68, moved out of the palatial country home that he shared with Catherine, 43, and their two young children, the sources add.

“BETWEEN MICHAEL’S MEDICAL is­sues, Catherine’s mental health and Cameron’s legal troubles, they appar­ently couldn’t always live together,” a source told The ENQUIRER.

“Michael moved into a rental house for a short period. But their problems could have led to a permanent split and even a divorce.”

The couple, who is worth a combined $300 million, married on Nov. 18, 2000, and had plans to renew their wedding vows on their 10th anniversary. But then Michael was diagnosed with Stage 4 throat cancer in August 2010.

After undergoing an aggressive regimen of radiation and chemo­therapy, the “Basic Instinct” star an­nounced in January 2011 that he was cancer-free. But just three months later, Catherine checked herself into Silver Hill Hospital in New Canaan, Conn., where she was treated for bipolar II disorder, a condition that is triggered by stress.

“Catherine took drugs to help treat her bipolar condition, and although the medication may have helped her, they also may have played a role in the couple’s problems,” said the source.

Then in December 2011, Michael’s 34-year-old son Cameron – who was already in prison serving a five-year sentence on drug charges – was sentenced to another 4 1/2 years for possessing drugs while in jail.

“Dealing with all these issues put an additional strain on Michael and Catherine’s marriage,” said the source.

So last summer the “Fatal Attrac­tion” star left the $5 million mansion in Bedford, N.Y., where he lived with Catherine, son Dylan, 12, and daugh­ter Carys, 9, sources say.

“Michael stayed in a nearby rental property owned by a multimil­lionaire pal,” said an insider. “It’s a nine-room converted barn that’s just a few miles from the estate he shared with Catherine.”

Another source confirmed: “Mi­chael said something to the effect that he and Catherine were taking a ‘time-out,’ and he was looking for­ward to having an entire house as his personal man cave.”

BUT he didn't appear to be happy with Catherine’s behavior. In June, singer Charlotte Church told a Brit­ish talk-show audience that Michael seemingly got furious when she and Catherine partied at a celebrity golf tournament.

“We were singing and having a great time, and Michael was really ****** off,” Charlotte said.

Despite the couple’s problems, “Catherine was determined to save the marriage because she still loved Michael,” said a source. “So she brought the family together for the children’s sake.”

The couple celebrated the Fourth of July with their kids at a private par­ty in Malibu, and Michael, Catherine and Dylan were photographed going out to lunch in the ritzy en­clave on July 12. The entire family flew out of Los Angeles together on July 17, then attended a magic show at the Flamingo in Las Vegas on Aug. 26.

The couple’s hush-hush separation nearly became public knowl­edge when the talent­ed “Chicago” beauty missed a new screen­ing of her father-in-law Kirk Douglas’ 1960 classic “Sparta­cus” in Beverly Hills on Aug. 13, as well as a family dinner at the restau­rant Crustacean afterward.

“Catherine was scheduled to at­tend the screening, and everyone was asking where she was,” a source told The ENQUIRER.

“Michael made some lame excuse, but when she skipped the fam­ily dinner it was obvious that something was not right. When a close pal asked Michael about it, he said, ‘You know Catherine. She’s go­ing through one of her phases again.’

“I think he felt com­pletely betrayed that Catherine didn’t show up. But maybe she was upset with him and wanted to make a point by snubbing him on this special family evening. It must have been like a slap in the face to Michael.”

While reps for the couple insisted Welsh-born Catherine attended the dinner, eyewitnesses claimed she was not there, and Michael was photographed driving off alone.

As The ENQUIRER previously re­ported, Michael repeatedly cheated on his first wife Diandra, carrying on affairs with his “Romancing the Stone” co-star Kathleen Turner, British brewery heiress Sabrina Guinness and others.

The marriage reached its breaking point when Diandra caught Michael in bed with one of her friends at the Regent Beverly Wilshire hotel, where he’d checked in under Diandra’s maiden name as “Michael Luker.”

After Diandra ordered him to get help or get out, Michael underwent rehab for alcohol, sex and cocaine addictions at the Sierra Tucson clinic in Arizona. But it wasn’t enough to save the union.

And for a while, it looked like his marriage to Catherine was head­ing in the same direc­tion. But the two have worked out their dif­ferences, sources say, and they appeared together at the pre­miere of Catherine’s movie “Side Effects.”

At Catherine’s request, Mi­chael has also bought a home in Bedford for her mother, Patricia Jones, The ENQUIRER has learned.

Michael splashed out the full ask­ing price of $1.9 million for the 4,700-square-foot, 10-room home on more than four acres, sources say.

Now both stars are continuing to work on their marriage for the sake of their children, insiders say.

“It’s because of Dylan and Carys that Michael and Catherine aren’t giving up on each other,” said a source. “But only time will tell if they can make things work over the long haul.”