The 83-year-old actress was David's step-mother whilst she was married to his father Jack Cassidy from 1956 to 1974, and also played the late star's on-screen mother on 'The Partridge Family' in the 1970s.
And since David tragically passed away this week in hospital at the age of 67 after suffering from organ failure, Shirley has paid tribute to her late former step-son.
In a statement, Shirley said: ''Long before he played my son on 'The Partridge Family', David Cassidy was my step-son in real life. As a little boy, his sweet sensitivity and wicked sense of humour were already on display, and I will treasure the years we spent working and growing together. I will also find solace knowing that David is now with his dad.
''My heart is with David's daughter, Katie, his son, Beau, and with Shaun, Patrick, and Ryan, whom I know are deeply missing their brother today. Thank you for the enormous outpouring of affection you have offered our family at this difficult time.''
David passed away on Tuesday (21.11.17) after having been rushed to hospital last week, and his family confirmed the news in a statement.
They said: ''On behalf of the entire Cassidy family, it is with great sadness that we announce the passing of our father, our uncle, and our dear brother, David Cassidy. David died surrounded by those he loved, with joy in his heart and free from the pain that had gripped him for so long. Thank you for the abundance and support you have shown him these many years.''
Shirley isn't the only star to have paid tribute to the 'Daydreamer' singer either, as Brian Wilson, Marie Osmond and Harry Connick Jr. also shared their condolences.
Beach Boys singer Brian Tweeted: ''I'm very sad to hear about David Cassidy. There were times in the mid-1970s when he would come over to my house and we even started writing a song together. He was a very talented and nice person. Love & Mercy to David and his family.''
Whilst former The Osmonds star Marie hared a series of pictures from magazine covers and wrote: ''Heartbroken over the passing of #DavidCassidy. He graced the covers of teen magazines w/ my Brothers in the '70s. My condolences to his Family. #RIP #PartridgeFamily #DavidsHitSong #70sIcon #IWokeUpInLoveThisMorning (sic)''
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