Muhammad Ali is being treated in hospital for a respiratory condition, his spokesman has confirmed to Press Association Sport.
The 74-year-old former world heavyweight champion is being treated by doctors as a precaution, and a brief hospital stay is expected.
Ali, who suffers from Parkinson's Disease, was last admitted to hospital in January 2015 with a severe urinary tract infection.
Bob Gunnell, Ali's media representative, declined to say where the American is being treated.
Ali, who retired from the ring in 1981, is widely considered to be one of the greatest heavyweights in boxing history.
"He is being treated by his team of doctors and is in fair condition," Gunnell said in a statement from Phoenix, where Ali lives. "A brief hospital stay is expected. At this time, the Muhammad Ali family respectfully requests privacy."
The Associated Press have reported that Ali's condition may be worse than it has been in previous hospital visits. The boxing legend has been admitted to hospital several times, most recently in 2015 when he was diagnosed with a severe urinary tract infection initially diagnosed as pneumonia.