The ex glamour model said the teen idol, who died this month, forced himself on her when she was 19 after asking her to appear in one of his music videos.
Fox alleges that incident occurred following a glamour shoot the two did in 1985, where she posed topless.
Fox alleges that after the shoot Cassidy asked her to dinner, as well as her father, and at the restaurant when Fox got up to go to the bathroom Cassidy followed her. "But instead, he just stuck his tongue into my mouth and shoved a hand under my skirt", she said.
Speaking exclusively to Daily Star Sunday, she described him as a "first class creep" and said: "It shocked me".
Bosses at Rodier & Rodier originally sued The Partridge Family star in 2013 over unpaid legal fees amounting to £100,300.
'I had a big poster of David Cassidy on my wall and I used to kiss him every night before going to bed.
'When he grabbed me and pushed his tongued down my throat, I just kneed him in the b****ks and would still do that today'.
David Cassidy died after a battle with alcoholism which saw him arrested for DUI in 2013. Source Getty
Cassidy - who died earlier this month of liver failure - reportedly invited Fox and her father to dine with him after he and Fox had wrapped filming the music video for Cassidy's 1985 single, "Romance".
"I reacted quickly and instinctively by bringing my knee upwards, striking him right in the balls".
However, as Fox explains in her upcoming book, she was already disgusted by Cassidy's behaviour during the shoot and only agreed to have dinner with the star to ensure her father didn't suspect anything.
She says she then elbowed him in the face, and returned to the table.
"David seemed to be deliberately drawing out the photoshoot, which took an eternity, and he had an erection the entire time - something he did absolutely nothing to hide", she said.
But revealing in her autobiography, released next week, Fox said she responded by kneeing him in the crotch. In that moment, the handsome, sweet guy from The Partridge Family had transformed into a first-class creep who wasn't worth the risk of a stretch in prison.
Tata Motors announces price hike across passenger vehicles range, starting January 2018
Tata Motors shares traded flat after the announcement at Rs 411.50 a piece on BSE Sensex intraday. The company had launched 10,000 NEXON models last month from its Ranjangoan facility.
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