Mackenzie Phillips has confessed to having a consensual sexual relationship with her own father, The Mamas & The Papas singer John Phillips.

The former child star makes the horrifying revelation in a new autobiography, alleging the affair began when she was raped by the California Dreamin' star aged 19, on the eve of her wedding to Jeff Sessler.

MacKenzie Phillips in 2005

She writes in High on Arrival: "On the eve of my wedding, my father showed up, determined to stop it (the wedding). I had tons of pills, and Dad had tons of everything too. Eventually I passed out on Dad's bed.

"My father was not a man with boundaries. 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."

John Phillips (far left) with The Mamas and The Papas

The American Graffiti star's addiction spiralled out of control in the following years and even a shared stint in rehab - with her father - failed to put an end to their drug use.

The incestuous relationship later became consensual as the dad and daughter began touring in a band called the New Mamas and the Papas.

MacKenzie Phillips with her partner in 2005

She adds: "I was a fragment of a person, and my secret isolated me. One night Dad said, 'We could just run away to a country where no one would look down on us. There are countries where this is an accepted practice. Maybe Fiji.' He was completely delusional.

"'No,' I thought, 'we're going to hell for this.'"