Best, who needed a liver transplant three years ago after years of alcohol abuse, is in a stable but critical condition on life support in the Cromwell Hospital in west London.
The 59-year-old former Manchester United star has a lung infection after being hospitalized for the past six weeks with a series of infections. He had appeared to be improving until Thursday night.
"He is holding, that's what I can say. His condition really hasn't changed since yesterday," said Professor Roger Williams, who is the senior doctor of the team treating Best.
"But I would like to have seen some very definite improvement in this last 24 hours. So I think the next 24 hours is going to be critical as to whether he's going to make it or not."
Doctors had stopped the internal bleeding that left him close to death three weeks ago. He was sitting up and talking to friends and family until his latest relapse.
Members of Best family have been at the hospital for the past few days, but Williams said Best remained heavily sedated and was not communicating with those around him.
"There's the usual team of nurses and doctors and the family are in the waiting room," he said. "They've said all sorts of rude words to him, good words, swear words, Irish words, everything to poke a reaction from him."
Regarded as one the most talented players of all time until his retirement at age 26, Best was renowned for his standout dribbling skills and memorable goals and helped Manchester United win the European Cup in 1968.
Best's career was marred by his wayward lifestyle - his womanising led to a divorce from second wife Alex a year ago - and he continued to drink heavily after his liver transplant.