His son Beau, 26, wrote on the star's official Facebook page today to update concerned fans about his dad's condition.
He said: "Unfortunately David is very sick. However he is getting the support he needs, surrounded by the people he loves most.
"Thank you very much for your love and concern that you have expressed in your messages to him."
Cassidy, who is best known for his starring role as Keith Partridge in the 1970s series The Partridge Family, is understood to have suffered multiple organ failure.
A source told TMZ.com that the star's condition was "grim" and his family are being told they "need to prepare for the worst" as loved ones, , including his son, ex-wife Sue and brother Patrick, surrounded his bedside.
Another source told Us magazine he was taken to hospital three days ago suffering from kidney and liver failure.
They said: “He is hoping and waiting for a liver transplant, if he happens to get one in time, the prognosis is still up in the air.
“He was in an induced coma, but as of right now is conscious. He is on pain medication to keep him comfortable. He is in out of consciousness.
“He is receiving an outpouring of support and everyone is praying for his recovery.”
He has reportedly been suffering health problems for two months.
Earlier this year, Cassidy revealed he is battling dementia and planned to stop touring to concentrate on his health.
He confirmed he was fighting the disease after he was filmed slurring and falling off stage during a gig.
“I want to focus on what I am, who I am and how I’ve been without any distractions. I want to love. I want to enjoy life.”
Cassidy, who has suffered from alcohol problems in the past, also said he had decided to stop touring after struggling to remember lyrics to his classic songs.
As well as health issues, Cassidy also suffered a string of legal problems in recent years, including several arrests on suspicion of drink-driving, and a court-ordered rehab stint in 2014.
The actor, who received an Emmy nomination for his appearance on Police Story in 1978, was most recently seen on TV in a 2013 episode of CSI: Crime Scene Investigation.
Many ESC fans from all over the world are so very sad because we lost Joy Fleming - one of the best singers ever.
Betty MacDonald fan club founder Wolfgang Hampel sings 'Try to remember' especially for Betty MacDonald fan club organizer Linde Lund at Vita Magica September
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