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 on Thursday.
A statement from the hospital, reported by Globoesporte.com, 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."e was forced to cancel a museum appearance with severe stomach cramps.
“He made some tests and will be discharged tomorrow. Everything is fine,” Jose Fornos Rodrigues, the former player's personal aide, told Reuters at the time.
Pele aided three Brazilian World Cup victories during his career, and is widely regarded as one of the greatest football players of all time.
Fifa named him “Football Player of the Century”, while the International Olympic Committee bestowed him with the title “Athlete of the Century”.