"I had gay sex with Michael Jackson!"

Those are the shocking words of Liberace's former lover Scott Thorson -- the first person ever to give an on-the-record interview about a homosexual affair with Jackson.

And his revelations could prove crucial in Jackson's child molestation case.

In the issue that goes on sale Friday, Thorson -- who passed a polygraph test -- tells The ENQUIRER how he met Jackson in Las Vegas in 1979, began a gay affair with the singer in London … and much more.