Football legend George Best is in a "serious but improving" condition after being admitted to hospital suffering from a kidney infection.
George Best received a liver transplant in 2002
The former Manchester United and Northern Ireland player is being treated in the intensive care unit of London's Cromwell Hospital.
Best, 59, was admitted with flu-like symptoms but became "seriously ill" and is being treated with antibiotics.
Professor Roger Williams said Best's condition was not alcohol-related.
Since a liver transplant in 2002 he has been more susceptible to infections.
Professor Williams, who has been responsible for Best's care since the transplant, said Best was responding well to treatment since admission on Saturday.
"He is serious but improving, with the emphasis on improving," he said.
GEORGE BEST'S HEALTH
March 2000: Severe liver damage diagnosed
February 2001: Treated for pneumonia
April 2001: Anti-alcohol pellets implanted into his stomach
July 2002: Undergoes liver transplant
November 2004: Routine operation to check on liver transplant
October 2005: Treated for kidney infection in intensive care
"The infection has caused him to be severely ill but he is certainly responding to treatment and we hope to move him out of intensive care in the next 24 hours."
Prof Williams told BBC Radio Five Live that the former footballer's current illness was not "directly related" to his alcoholism, despite admitting concern at Best's failure to give up drinking.
But this notion has been disputed by the National Kidney Research Fund.
A spokesman said Best's present condition "could well be the result of taking a combination of immunosuppressive drugs, drugs to curb his desire for alcohol, and possible alcoholic consumption".
But he went on: "It is highly unlikely his kidney infection was caused solely by taking the drugs he was given to prevent him drinking."
Best's agent Phil Hughes added that the star was "responding to treatment", although he was "still very, very ill".
"His kidneys are showing signs of improvements," he said.
"After a few tests they decided that it was his kidneys that were infected and this was due to the medication he's been taking for his liver.
"His function tests are showing improvements so we're keeping our fingers crossed".
Leaving the hospital on Monday, Best's son Calum told waiting reporters: "He's OK. He's stabilised."
Despite warnings that continuing to drink after the transplant could kill him, Best failed to give up alcohol.
In November 2004, George Best declared himself "fit and healthy" after undergoing a routine operation to check on his liver transplant.
Earlier last year he was banned from driving for 20 months after pleading guilty to drink-driving.
His marriage to his former air hostess wife Alex ended in a "quickie" divorce a few months later.
His transplant had prompted questions about whether alcoholics should get donor livers, and Best's doctor warned that continued drinking would be catastrophic.
Best received the transplant at the private Cromwell Hospital, which specialises in liver disease and transplants.
He has visited the hospital several times since, either for check-ups or because of infection.