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UPDATE – 10:18PM

“I had sex with my own father.”

Mackenzie Phillips will reveal tomorrow on Oprah that her father, Mamas and Papas founder John Phillips, raped her, Gossip Cop has learned. The interview coincides with the release of her memoir, “High on Arrival.” Already it’s been revealed in promos for tomorrow’s program that her father introduced the “One Day at a Time” star to drugs. She recalls, “My father shot me up for the first time.” She also discusses Mick Jagger‘s attempt to seduce her and other long-held secrets.

But the most stunning revelation is that John had sex with a drugged-out 19-year-old Mackenzie as she faded in and out of consciousness on the eve of her wedding to Jeff Sessler in 1979. According to a leaked copy of the book, when she later confronted him about it, she said, “We have to talk about how you raped me,” to which he responded, “You mean when we made love?” The two had a long-term incestuous relationship, the memoir reveals.

Earlier this afternoon Oprah tweeted, “Interviewed One Day at a Time’s Mackenzie Phillips. Thought I’d heard it all … beyond shocking! We’re airing it Wednesday.”

We contacted Harpo Productions, and they did not get back to us. We also exchanged a series of e-mails with Simon & Schuster, the memoir’s publisher, explaining that we had heard the book’s big secret was that Mackenzie and John Phillips had a long-term sexual relationship. We have now confirmed it.