The former Manchester United footballer has spent three weeks in the private Cromwell Hospital in west London after being struck down with an infection. Best, 59, who also played for Northern Ireland, had a life-saving liver transplant in 2002 following well-documented drinking problems.
Earlier this week his agent Phil Hughes said he was "doing very well" and his condition had improved.
But last night it appeared that his health had taken a turn for the worse. It was understood that he was suffering from internal bleeding and was on life support.
His former wife Alex said: "I have been told that George's condition has deteriorated dramatically during the day. I am just praying that, once again, he somehow manages to pull through against all the odds. My thoughts and prayers are with him."
Professor Roger Williams, Best's long-term doctor, said: "It may be an exaggeration to say that Mr Best is gravely ill. But he is certainly severely ill and is fighting for his life. We are not giving up. There's still a chance we can get him through this." He said he had not traced the cause of the bleeding.
Last weekend, Best's girlfriend, Ros Hollidge, said his three sisters had flown to England to be at his side.
"George is in a critical condition," she said. "He's not necessarily in the clear. He looks very frail and it's not good. His sisters have made a big effort to fly over."
Best was admitted to the hospital with flu-like symptoms and developed a kidney infection. Professor Williams said that prescribed drugs needed after his 2002 liver transplant, rather than alcohol abuse, were the cause of his illness.
He said the ex-footballer had become more susceptible to infections after a course of medication to suppress the immune system and prevent his body rejecting the new liver.
At the time of his admission, Professor Williams would not say whether Best had been drinking again, although reports of Best's failure to give up the bottle following his operation three years ago had raised fears. "We're disappointed, there's been these awful bouts of drinking which have gone on and he's had all these other sorts of social problems," said Professor Williams.
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