Crown Prince Albert of Monaco has taken over the royal duties of his seriously ill father, the palace has said.
Prince Albert has been described as a reluctant heir
Prince Rainier, 81, has been in hospital since 7 March and was taken into intensive care a week ago.
The palace said Prince Albert would take over as his father was no longer able to exercise his royal functions.
Prince Rainier, who is being treated for heart, lung and kidney problems, came to the throne in 1949 and is one of Europe's longest reigning monarchs.
The health updates from doctors at the hospital this week have not been optimistic, with reports that the prognosis was "extremely guarded".
On Wednesday, his health was described as stable but fragile.
The statement from the royal palace said the Council of the Crown had decided that it was difficult for Prince Rainier to "exercise his high functions".
"From now, the regency is assured by His Serene Highness Prince Albert," it said.
Prince Albert, 47, is the only son of Prince Rainier and his late wife, American actress Grace Kelly.
He is not married and has no children - which reportedly caused such concern that Monaco's constitution was revised to ensure the continuation of the dynasty, through his sisters.