Prince Rainier of Monaco is on an artificial respirator suffering heart and kidney trouble but his condition is stable, according to the palace.
Prince Rainier's health has taken a turn for the worse
The 81-year-old has been in intensive care since Tuesday after a lung infection deteriorated suddenly.
He has had heart and respiratory problems for the past decade and was in and out of hospital in recent months.
Rainier III, who acceded to the small principality's throne in 1949, is one of Europe's longest-reigning monarchs.
The prince entered hospital on 7 March and had appeared to be getting better but his health took a turn for the worse at the start of the week.
A palace communique said the prince was being treated for a lung ailment complicated by a heart condition.
He suffered a relapse of a lung infection which, together with a weak heart and kidneys, required him to be moved to intensive care.
"His breathing troubles made it necessary to use artificial respiratory assistance," the communique added.
His children Prince Albert and Princesses Caroline and Stephanie went to see him in the intensive care unit at the Monaco Cardio-Thoracic Centre after he was placed there on Tuesday.
Prince Rainier made world headlines with his marriage to Hollywood movie star Grace Kelly in 1956 after meeting her at the Cannes film festival.
He has been credited with reducing Monaco's reliance on tourism and gambling by developing the business sector.
However his advancing age and ill-health have led him to delegate more and more responsibility to his son and heir to the throne, Prince Albert.