Prince Rainier of Monaco is in a "worrying" condition in intensive care, and the prognosis is "uncertain", the palace has announced.
Prince Rainier has delegated much power to his son
The 81-year-old prince has been on a respirator since Tuesday when a lung infection became more severe.
He has had heart and respiratory problems for 10 years and has been 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.
He went into 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.
The statement from his doctors said: "Due to the weakness of his cardiac, respiratory and kidney functions, the prognosis of the sovereign's health is uncertain.
"The state of health of his Serene Highness, Prince Rainier III, who is still in intensive care under artificial respiration in Monaco's cardio-thoracic centre, remains worrying."
The statement added that Dr Jean-Charles Piette, chief of internal medicine at La Pitie Salpetriere hospital in Paris, had been consulted about the prince's condition.
Prince Rainier's three children with his late wife, US actress Grace Kelly, - Crown Prince Albert, 47, Princess Caroline, 48, and Princess Stephanie, 40 - have been at his bedside.
French President Jacques Chirac phoned Prince Albert on Friday to express his concern and to inquire about his father's condition.
A earlier 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.
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.