Robin Gibb’s family was keeping a bedside vigil, praying for the 62-year-old’s survival.
The Bee Gees star appeared to have made a recovery from liver and colon cancer this year, but doctors believe a second tumor may be present, reported The Sun.
He has also developed pneumonia.
Robin Gibb’s wife, Dwina, brother Barry, 65, daughter Melissa, 37, and sons Spencer, 39, and Robin-John, 29, were at his bedside at a private hospital in Chelsea, West London.
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 Gibb revealed his battle with cancer in October 2010.
He had emergency surgery to treat a blocked bowel, before a further operation to treat a twisted bowel. Colon cancer was then discovered and it spread to his liver.
Robin Gibb’s twin brother Maurice died of complications resulting from a twisted intestine in 2003, aged 53.
Robin Gibb famously appeared on the Alan Titchmarsh Show looking pale and gaunt and has cancelled a handful of appearance over the past two years because of his health.
Barry Gibb earlier this week jetted into the UK from Tennessee in the U.S. to join other members of the family.
His brother’s arrival this week comes after his nephew – Robin Gibb’s son – RJ described the overwhelming moment the family learned that the Bee Gees star was in remission from cancer.
RJ, 29, full name Robin-John, revealed that he was sitting next to his famous father in his hospital bed when the doctor told them the happy news.
“I leant over the bed and I kissed him on the forehead and just said, <<I love you>>,” RJ told the Mirror.
“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 much better.”
However, this isn’t the first time that Robin Gibb’s family and fans have been left worried as the singer also re-entered hospital back in February.
Robin Gibb was admitted into a London clinic as part of his treatment and rehabilitation.
At the time a family source told The Sun: “We feared the worst. But doctors have said they are astounded at his response to treatment.”
But it was only a matter of weeks before this that Robin Gibb insisted he had made a “spectacular” recovery.
Speaking to BBC Radio 2 host Steve Wright, the artist revealed that he felt better than he had done for a decade.
“The prognosis is that it’s almost gone and I feel fantastic and really from now on it’s just what they could describe as a <<mopping-up>> operation,” he said.
“I am very active and my sense of well-being is good.”
Robin Gibb also suggested that reports about his health had been conjecture, adding: “I mean the fact is, I’ve never spoken to anybody about my condition or the condition that I was in and a lot of them go over the top to the point where they’re telling me things that I didn’t even know about myself.”
And despite being admitted to hospital last week for further intestinal surgery, the singer had been given the all clear.
A statement released on the singer’s behalf said: “On Sunday 25 March, Robin Gibb underwent further intestinal surgery.
“He is currently recovering in hospital and therefore, for the time being, all existing commitments prior to the Titanic Requiem concert, have had to be cancelled.”
Robin Gibb and RJ had composed the music for the Royal Philharmonic performance at Westminster Central Hall next week.
In February he had talked about feeling “fantastic” following his treatment and said he had begun to put on weight after looking increasingly gaunt.
At first his illness was thought to have been due to the hereditary intestinal condition which led to the death of his twin brother.
Speaking about his cancer, last month he said: “The prognosis is that it’s almost gone and I feel fantastic and really from now on it’s just what they could describe as a <<mopping-up>> operation.”
In an interview earlier this month, Robin Gibb pondered whether his illness is “karma” for the fame and fortune he has enjoyed.
“I sometimes wonder if all the tragedies my family has suffered, like Andy and Maurice dying so young and everything that’s happened to me recently, is a kind of karmic price we are paying for all the fame and fortune we’ve had.
“But we’ve worked hard for everything we’ve achieved.”
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