David Cassidy is in a coma and is suffering from multiple organ failure, according to reports.
The 67-year-old Partridge Family star and singer was rushed to hospital in Florida three days ago, the Daily Mail reports.
A source told TMZ that his condition was "grim". He is believed to have been in failing health for the last two months.
Doctors believe he could die if he doesn't get an urgent kidney or liver transplant.
His family have been told to prepare for the worst. The gossip site says they have gathered at his bedside.
A source told The Blast he is "in and out of consciousness" and his condition is being monitored very closely.
Cassidy is widely known for his starring role as Keith Partridge on the 1970s series The Partridge Family.
In February, the star revealed he was battling dementia after he fell on stage during a concert in Los Angeles. His mother, actress Evelyn Ward, and his grandfather also had the condition.
Fans were concerned for his well-being when he struggled through the gig in February.
He stumbled on stage and forgot some of his words, prompting fears he had started drinking again.
He told People magazine after the gig: "I was in denial, but a part of me always knew this was coming."
His mother the actress Evelyn Ward struggled with dementia until her death at 89 in 2012.
"In the end, the only way I knew she recognized me is with one single tear that would drop from her eye every time I walked into the room. ... I feared I would end up that way," Cassidy said.
"I want to focus on what I am, who I am and how I've been without any distractions," he added. "I want to love. I want to enjoy life."
Video emerged of the one-time teen heartthrob over the that showed him slurring, taking long breaks, telling rambling stories and even falling over at one point.
He told the audience he will no longer be touring.
While he vowed to make the show his last, he struggled to stay on his feet and play the guitar.
Issues in Cassidy's personal life have been well-documented over the last few years.
He has filed for bankruptcy, divorced from third wife Sue Shifrin, has had multiple arrests, eye surgeries and went through a stint in rehab.
His mother, the actress Evelyn Ward, struggled with dementia until her death at 89 in 2012.