The 62-year-old singer’s family have been keeping vigil at his bedside at a hospital in Chelsea, west London.
They are understood to include his wife Dwina, brother Barry, 65, daughter Melissa, 37, and sons Spencer, 39, and Robin-John, 29.
Gibb had surgery on his bowel 18 months ago for an unrelated condition, but a tumour was discovered and he was diagnosed with cancer of the colon and, subsequently, of the liver.
It had been thought his cancer was in remission as early as last month, but the latest deterioration in his health coincides with reports of a secondary tumour.
Sources close to Gibb confirmed he is very ill, but his agent declined to comment on reports that the star may have only days left to live. Gibb’s twin brother and bandmate, Maurice, died from the same bowel condition that initially led doctors to operate on Robin.
Gibb’s younger brother, Andy, who was not part of the Bee Gees but a successful singer in his own right, died in 1988 from heart failure at the age of 30.
Robin Gibb has enjoyed a musical career spanning six decades, from humble beginnings as part of a sibling trio in 1950s Manchester to his most recent classical venture, a requiem for the Titanic.