He is near death, ABC News reported, although a message on his Web site is positive.
"We are all hoping and praying that he will pull through," his Web site said.
Gibb, 62, who said in February he was making a recovery from cancer, was later readmitted to the hospital for intestinal surgery.
Gibbs' wife, children and his brother Barry have been keeping vigil at his bedside at a hospital in Chelsea west London, the BBC reported Sunday.
Doctors discovered cancer of the colon and liver during bowel surgery 18 months ago, but it was thought his cancer was in remission.
However, a secondary tumor was found, The Daily Telegraph said.
A message on the homepage of his official RobinGibb.com Web site Sunday read: "Because of this situation, Robin's Web site is temporarily unavailable. Sorry for any inconvenience."
Gibb's Facebook page remains online and fans have been leaving messages of support, the BBC reported.