The palace issued a statement Friday saying: "He is under continuous surveillance. His condition is serious and requires permanent medical attention. ... Biological and radiological tests as well as a scan have been carried out to determine the scope of the ailment."
Officials have refused to confirm or deny reports the prince was in a coma.
The prince, 51, was admitted to Princess Grace Hospital Tuesday, the day before his father-in-law, Prince Rainier, died of multiple organ failure, hellomagazine.com reported.
Prince Ernst is a direct descendant of Britain's King George III and married Caroline in 1999.
Celebrity Families of 2014 [PHOTOS]