A frail Pope John Paul II has sent a special blessing to Monaco's Prince Rainier, who remains on life support.
Palace officials said Prince Rainier's condition was "worsening", in a statement issued on Saturday.
Doctors are reserving their prognosis for the 81-year-old prince who has been on a respirator since Tuesday when a lung infection worsened.
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 prognosis remains extremely guarded"
"The state of health of His Serene Highness Prince Rainier III is continuing to worsen," the palace bulletin said.
"Despite the best available care and the control of the heart and lung infection, his heart, lung and kidney functions are gradually deteriorating," the palace bulletin said.
"The prognosis remains extremely guarded," it added.
The Pope, who has been absent from Vatican ceremonies for Easter week because of his own ill health, wished Prince Rainier well.
"Informed of the health problems affecting Your Serene Highness, the Holy Father joins with you in thought and prayer with warm wishes towards you," said the message signed by papal secretary of state Cardinal Angelo Sodano.
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.
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An 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 and be put on a respirator.
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.
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