Monaco's Prince Albert has assumed the duties of his sick father, Prince Rainier, as monarch of the tiny state on the Mediterranean coast.
A brief statement from the palace did not say whether Prince Albert (47), who is in line to succeed his father, was taking over the monarch's duties permanently.
In a separate statement, Albert said his father's health remained "very fragile".
Prince Rainier (81), is in intensive care in hospital after suffering lung, heart and kidney problems. He is one of the world's longest-serving monarchs and was married to the late film actress Grace Kelly.
"In accordance with the statutes of the sovereign family, the (advisory) Crown Council took note of the inability of His Most Serene Highness Prince Rainier III to exercise his functions," the palace said in the statement.
"Prince Albert is empowered to exercise sovereign powers in the name of the sick sovereign prince," the palace added.
In a written message to the people of Monaco, Albert said he would dedicate himself to carrying out his father's duties. "I would like to assure you that I will devote myself to this with strength, conviction and passion," he said.
Monaco's National Council, which governs the tax haven, said in a statement it would give Albert its full support.
He has been hounded less by paparazzi than his father and mother and his glamorous sisters, Princess Caroline and Princess Stephanie. Albert has been called Europe's most eligible bachelor.
Rainier swore never to abdicate as long as his son was unmarried and childless. Albert was educated in Monaco and the United States, where he majored in political science, and served in the French Navy.
Sport remains his passion. He dreamt of a career in soccer, took part in the Paris-Dakar motor rally in 1985 and is a judo black belt.
The Grimaldi family has ruled the principality for seven centuries. Rainier has been in and out of hospital in recent years with lung and heart problems, and was most recently admitted on March 7th. He was born on May 31th, 1923.
Princess Grace was killed in a car crash in 1982.