The Bee Gees star - who has been battling cancer in the last few years - is suffering from pneumonia, and his family, including wife Dwina and brother Barry, are keeping a vigil at his bedside at a private hospital in Chelsea, London, with doctors thinking he may only have a few days to live.
A family friend told The Sun: "Our prayers are with Robin. He has kept so positive and always believed he could beat this.
"Sadly, it looks like he has developed pneumonia, which is very bad in his situation.
"If there is anyone you would put money on pulling through such a dire situation, it would be Robin because he is a fighter. But this is a battle he will struggle to win."
Robin, 62 - who starred in the Bee Gees alongside siblings Maurice and Barry - was recently thought to be in remission from his cancer but doctors are now believed to have found a secondary tumour.
Just last month he said he was excited to be back after his health problems and was keen for his fans to hear some new work from him.
He said: "I can`t wait to carry on with my work. I want all my fans to know their love helps me so much."