LOS ANGELES – Michael Jackson’s deepest sex secrets could spill out in his bitter $3.8 million court battle with his former video producer, a bombshell court document reveals.

The trial, set to kick off today, pits Marc Schaffel against the pop star in a fight over money and alleged broken deals during their three-year work partnership that ended in 2004.

But it could become a sex-soaked mud-slinging fest, according to a trial brief obtained by The News.

Schaffel may testify about “Jackson’s sexual proclivities” and “intimate details of their conversations” if the pop star’s lawyers try to slime him for his past as a porno-film director and producer, the brief, penned by Schaffel’s lawyer Howard King, says.

“Jackson’s primary response to all of the claims will be an attempt to smear Schaffel using … prejudicial evidence regarding his background in the adult entertainment business and the sexual orientation of some of his films,” the document says.

Schaffel has admitted he directed and produced porn, including some gay-oriented flicks, before he began working for Jackson in 2001. Jackson’s lawyers are also expected to dredge up “innuendo and gossip” about Schaffel’s business past, the brief says.

If that happens, Schaffel will let loose. “If they go that route, and it appears they are going to, then Marc is going to tell what he knows about Michael’s interests,” a source close to Schaffel told The News.

During his child-molestation trial last year, prosecutors showed piles of hard-core heterosexual porn magazines and DVDs seized from Jackson’s Neverland Ranch. There were also a few “art” books featuring photos of naked and semiclothed young boys.

But the source hinted there may be more to learn about the pop idol’s “tastes.”

Schaffel produced two TV specials to boost Jackson’s image after the singer was hit with kiddie-sex charges in late 2003.