The prince's health "does not stop worsening," despite treatment, the palace said in a medical bulletin signed Saturday by three doctors. "The prognosis for life remains extremely reserved," it said. A similar bulletin Friday did not use the word "extremely."
The previous bulletin from Rainier's doctors had said the prince remained in "worrisome" health after being hooked up to a respirator earlier in the week after suffering heart and kidney failure.
Doctors refused to say whether the 81-year-old monarch of one of Europe's longest-ruling dynasties would survive the illness.
Rainier, who has suffered ill health in recent years, was hospitalized at the Cardio-Thoracic Center on March 7 with a chest infection. He was transferred to the intensive care unit Tuesday when his health took a sudden turn for the worse.
Rainier's three children, Prince Albert and Princesses Caroline and Stephanie, rushed to his bedside. Albert, Rainier's only son and heir, appeared Friday on a palace balcony before subjects praying for the monarch's life.
Pope John Paul II — himself suffering from ill health — sent a special blessing to Rainier, the palace said Saturday, saying he was united with the prince "in thoughts and prayer" and offering his heartfelt wishes.
"Entrusting in the intercession of the Virgin Mary, the pope asks the Lord to give you comfort and the support of grace and accords the prince and his family a special apostolic blessing," said the pontiff's message, sent Friday.
Rainier, who has ruled Monaco since 1949, is beloved in the tiny principality he transformed into a modern and elegant enclave for the rich.
"Prince Rainier is a good man, and he is loved here," said Bruno Paillet, 41, a waiter sitting in a cafe reading a front- page article in the Monaco-Matin newspaper with the headline: "Rainier: Serious state confirmed."
"All we can do is hope for the best," he said, pausing to look up from his newspaper. "Oh, it's going to be sad, very sad."
Others in the principality said they hoped Rainier will pull through.
"He mustn't die. He should live to be 120, even 130 years old," said Mohamed Rahik, a street-sweeper working nearly the principality's marina.
Rainier's family, the Grimaldis, have ruled Monaco for more than 700 years. However, Albert — long groomed to assume Rainier's mantle as monarch — has not married, raising concerns about heirs. In 2002, Monaco changed its succession law to allow power to pass from a reigning prince who has no descendants to his siblings.
Albert, 47, would take the title His Serene Highness. Princesses Caroline, 48, would be next in line to the throne, followed by her son, now 20.
Rainier's movie-star wife, Grace Kelly, died in a car crash in 1982.