Three-time World Cup winner Pele has been admitted to a "special care" unit in a Sao Paulo hospital.
The 74-year-old former Brazil forward was thought to be improving, having been taken to hospital on Monday suffering with a urinary tract infection, but was transferred to the unit.
A statement from the hospital read: "The Hospital Israelita Albert Einstein reports that the patient Edson Arantes do Nascimento (Pele) has been admitted with clinical instability.
"To receive the best care, he has been transferred to be monitored in a special care unit.
"The hospital will update when there is new information."
Pele's personal assistant Jose Fornos, known as 'Pepito', explained he was moved to a more private room, with no visitors allowed.
"It will take between eight and 10 days before he can leave the hospital," he told Globoesporte.com.
Pele was discharged from the same hospital on 13 November after undergoing surgery to remove kidney stones.
Pele, often called the greatest soccer player in history, has suffered a long list of health problems in the past decade, including emergency eye surgery for a detached retina and a hip replacement.