NEW DETAILS: The ENQUIRER first reported months ago that Mackenzie Phillips was molested by her father John Phillips, the founding lead singer of The Mamas & the Papas!

That’s the explosive secret the 49-year-old former child actress reveals in scathing new memoir High on Arrival.

"I’ve always known I would write this book," Mackenzie said in press material obtained by The ENQUIRER about tell-all.

Dad John Phillips died in 2001 – and Mackenzie is now finally healed enough to talk about the details of her painful childhood and molestation that occurred on the eve of her first wedding to Jeff Sessler in 1979.

"My father was not a man with boundaries,’ Phillips writes in the book.

"He was full of love, and he was sick with drugs. I woke up that night from a blackout to find myself having sex with my own father. Had this happened before? I didn’t know. All I can say is it was the first time I was aware of it.

"For a moment I was in my body, in that horrible truth, and then I slid back into a blackout."

Mackenzie, who has been married three times and has a 22-year-old son, Shane, hit showbiz at age 12 in the 1973 movie American Graffiti. She went on to play rebellious teen Julie Cooper on the sitcom One Day at a Time. But problems with drugs and alcohol have plagued her for most of her life.

Her addictions led to the downward spiral of her career, arrests and many stints in rehab, as well as two near-fatal overdoses.

"In addition to the molestation disclosure,the book will certainly cover a wild life of sex, drugs and rock ‘n’ roll," added the source. "She’ll write about getting high with her father and rolling joints for the Rolling Stones." AND having sex with Mick Jagger at age 18.

"It’ll be an eye-opening memoir – and uplifting in the sense that Mackenzie has finally found redemption and peace in her life," said a publishing insider

And yet  Mackenzie’s step- mother Michelle Phillips (John’s second wife) dismissed the molestation charges in the book.

"You should take with a grain of salt anything that’s said by a person who has had a needle stuck in their arm for 35 years.

"The whole story IS disgusting."