Troubled CATHERINE ZETA-JONES is at the cen­ter of a bombshell drugs, booze and cheating scandal as her 12-year marriage to MICHAEL DOUGLAS disintegrates into a possible $300 million divorce battle.

The ENQUIRER has uncovered the shocking secrets behind the split – in­cluding vicious fights over Catherine’s dangerous habit of mixing alcohol with her bipolar disorder medications as well as cancer-survivor Michael’s “obsession” that his gorgeous raven-haired wife still carries a torch for her playboy ex-beau.

“In the end, something had to give,” said a friend. “They were driving each other crazy. Catherine’s drinking was making her even more despondent… and Michael couldn’t control his jeal­ousy. At his darkest hour, while he was battling throat cancer, he became con­vinced that Catherine still had feelings for her old boyfriend and was going to leave him.”

Following a series of ENQUIRER stories about the couple’s crumbling union, their reps finally admitted they were “taking some time apart to evalu­ate and work on their marriage.”

The cracks in the union began to ap­pear months ago – before Catherine was readmitted to a psych ward in April for her bipolar disorder. Most of their arguments centered around her drinking, according to an insider.

Their fighting got so bad that more than a year ago, Michael, 68, moved out of their $5 million country home in Bedford, N.Y., to live in a converted four-bedroom barn on the grounds of a friend’s $15 million estate.

The warring couple’s private battles have also spilled into their public life, as patrons and staff at a restaurant near their New York home recently witnessed a nasty blowout between them.

“Michael had a single glass of wine while Catherine was downing the hard stuff – mixing it up with a martini and then vodka on the rocks,” said an eye­witness. “Out of the clear blue, she erupted.

“It wasn’t a screaming match, but her voice was definitely raised and she pointed her finger at Michael. He ignored her, and that seemed to agitate her even more.

“After a few minutes, she finally calmed down and they sat quiet and stone-faced for the rest of their meal.”

Neighbors also claimed that the 43-year-old Welsh-born beauty made “a beeline for the liquor store” every time she hit town, stocking up on mul­tiple bottles of Grey Goose and ONE ROQ Vodka as well as Chardonnay.

It reached the point where Michael was worried for the mom-of-two’s safety.

“He knew she was drinking more and more,” said the source. “On top of that, she’s on medication for her condition – and drinking and drugs is a dangerous combination.”

While medical experts warned that Catherine faced a serious risk of self-harm or suicide because of her mental illness, an insider re­vealed that it’s Michael who has become more and more unhinged in recent months over fears that Catherine could get back with for­mer lover John Leslie. The one-time TV personality was famously at the center of a sordid sex scandal with accusations that he had fondled women. He also was accused of raping a TV weathergirl, but he did not face charges.

“Catherine and John had a scorch­ing 18-month affair in the early 1990s when she was 21 and he was 26, and they have stayed in touch over the years,” said the source. “He’s always called Catherine his ‘true love’ and refers to her as ‘the most beautiful woman I’ve ever seen.’ ”

During Michael’s battle with throat cancer, he apparently con­vinced himself that Catherine wanted to leave him.

“He couldn’t help thinking that he was too old for her and she wanted to get back with her old British love – or, in his worst fears, that she was already cheating with him,” said the source.

“In the end, it became too much to handle.”

If Michael and Catherine – par­ents to son Dylan, 13, and daughter Carys, 10 – decide to divorce, it’s Michael who would have the upper hand, says one top divorce attorney.

“If Michael can prove that Cath­erine’s had contact with an ex-lover (John Leslie) during the course of their marriage, and given his kinky sexual proclivities, this could be a deterrent to her in winning custody,” said famed New York matrimonial lawyer Raoul L. Felder. “Add to that her mental illness…exacerbated by drinking and pills, and she appears to be the more vulnerable party.

“If this matter can’t be worked out amicably between them and it ends up in a court case, there could be lots of bitterness, animosity and mudslinging. It could get ugly real fast.”