Bee Gee Robin Gibb is in a coma after suffering from pneumonia, his official website says.
He has been recovering from cancer in recent months, but was too ill to attend a performance of his new work a few days ago.
"We are all hoping and praying that he will pull through," his website said.
In February, Gibb, 62, said he had been making a "spectacular" recovery from cancer but he was later readmitted to hospital for intestinal surgery.
The message on his homepage added: "Because of this situation, Robin's website is temporarily unavailable. Sorry for any inconvenience."
Gibb's family have been keeping vigil at his bedside at a hospital in Chelsea, west London, the Press Association reports.
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BBC News - Robin Gibb in coma with pneumonia
Say what you will about 'disco' but these were uber-talented lads.