Former Bee Gee Robin Gibb is in a coma after contracting pneumonia, according to a statement on his official website.
The statement, which confirmed media reports the singer was gravely ill, said "we are all hoping and praying that he will pull through".
Family members, including his wife Dwina and brother Barry, were reportedly at Gibb's bedside in a London hospital.
Gibb's publicist, Doug Wright, declined to comment, but the musician's son has acknowledged the 62-year-old is seriously ill in a hospital.
Gibb was admitted to hospital last year for stomach and colon problems. He has not specified the exact nature of his illness - widely reported to be cancer - but told the BBC he had a growth on his colon that was removed.
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Earlier this year he said he was making a good recovery, but had intestinal surgery last month and was forced to miss the London premiere of his classical Titanic Requiem this week because of illness.
His son Robin-John Gibb said the family was "praying for him and hoping he has a speedy recovery".
The Bee Gees - UK-born, Australia-raised brothers Robin, Barry and Maurice Gibb - had a string of disco-era hits including How Deep is Your Love and Stayin' Alive.
Their soundtrack to the movie Saturday Night Fever was one of the best-selling albums of the 1970s.
Maurice Gibb, Robin's twin, died in 2003 within days of having a heart attack during an operation to remove an intestinal blockage.
