The 62-year-old Bee Gee's star's condition is said to be so serious that brother Barry, 65, has flown in from Tennessee, America, to make an emergency visit.
A source told The Sun newspaper: "They fear it may be the last time they'll see Robin. His recovery and determination has been remarkable, but there's been a turn for the worse.
"His body's taken a hell of a beating. The doctors fear for him but he's determined to battle on."
The teetotal vegan was first diagnosed with cancer in April 2011 after being admitted to hospital with abdominal pains, but just last month his son Robin-John - know as RJ - revealed the chemotherapy had wiped out all signs of the disease.
Talking about the moment he heard the news, RJ said: "I leant over the hospital bed and I kissed him on the forehead, looked him in the eyes and just said, 'I love you'.
"Then we sat together for ages, just taking it all in and being together without saying anything. It was the best news we could have hoped for.
"It was the most beautiful feeling and instantly life was so, so much better."
24.com publishes all comments posted on articles provided that they adhere to our Comments Policy. Should you wish to report a comment for editorial review, please do so by clicking the 'Report Comment' button to the right of each comment.
The Voice SA coach, Bobby van Jaarsveld, has released a new music video and we feel #blessed. Read More »
Australian progressive rock band Karnivool is heading to SA in April and we got to ask lead singer Ian Kenny a few questions. Read More »
All the deets.
Live gig pics.
New local act.
Our epic night.
JohannesburgFirst Time PlacementsR15 000 - R18 000 Per Month
RoodepoortPlacement Point (Pty) Ltd (Parkwood Branch)R6 000 Per Month
East RandPlacement Point (Pty) Ltd (Parkwood Branch)R12 500 Per Month
HousesR 4 990 000
HousesR 499 000
HousesR 1 669 000