A spokesman for the 62-year-old, who has been battling cancer, was not immediately available to comment on reports in the British media that Gibb had been surrounded by close family in a London hospital and may have only days to live.
“Sadly the reports are true that Robin has contracted pneumonia and is in a coma,” a statement on www.robingibb.com said.
“We are all hoping and praying that he will pull through.”
The website has been closed down temporarily.
An unnamed family friend told the Sun newspaper: “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.”
The tabloid said that Gibb’s wife Dwina, sons Spencer and Robin-John, daughter Melissa and brother Barry were keeping a bedside vigil.
In February, Gibb announced he had made a “spectacular” recovery from cancer, but in late March he underwent further surgery on his intestines.
He was forced to cancel all engagements, including the world premiere earlier this month of his first classical work, co-written with Robin-John, called “The Titanic Requiem”.
According to the Sun, Gibb had emergency surgery in 2010 to treat a blocked bowel and further surgery for a twisted bowel – the condition that killed his twin brother Maurice in 2003 at the age of 53.
He was diagnosed with colon cancer, which later spread to his liver.
Gibb was born in the Isle of Man between England and Ireland in 1949 with twin brother and fellow Bee Gees founder Maurice.
His family moved to Manchester in northern England and then Australia, where the twins, along with older brother Barry, began performing together.
The Bee Gees released their first record in 1963, but it was only in the 1970s that the brothers rose to worldwide fame, producing a string of disco favourites including “Jive Talkin'”, “Stayin’ Alive” and “Night Fever”.
The Bee Gees never matched that success in subsequent decades, although Barry in particular produced a string of hits for other artists including Barbra Streisand and Diana Ross.
The band’s distinctive tight harmonies and falsetto voices helped it sell an estimated 200 million albums worldwide, making it one of the most successful pop acts in history. AGENCIES