U.S. Tv icon Shirley Jones has given her onscreen son David Cassidy a big boost by revealing he's very well endowed in a leaked extract from her memoirs.

The actress, who played the 1970s teen hunk's mum in The Partridge Family, has opened up about her in-car sex romps with Cassidy's dad - her husband Jack Cassidy - and other salacious stories in the tome, Shirley Jones: A Memoir, and she reveals her Tv son was just as blessed as his real-life father.

In an extract leaked to the New York Daily News a month ahead of the book's scheduled release, Jones explains her real-life stepson was very well endowed, writing, "David’s brothers called him Donk, for Donkey."

The actress also details a crazy night in the late 1960s when the late Anthony Newley allegedly suggested Jones and her husband Jack wrap up a dinner party by getting naked and watching X-rated films with him and his then-wife Joan Collins.

The Partridge Family star also opens up about her lunchtime sex romps with her ex-husband, adding, "At the height of The Partridge Family, now and again Jack would pick me up from the studio in the car, then drive us into the garage adjacent to our house, where he would have intercourse with me in the backseat."