The health of Monaco's Prince Rainier is "precarious" and there is only a very slim chance that he will recover, the principality's royal palace says.
The prince has ruled the tiny Mediterranean state since 1949
"The hope for a favourable outcome remains extremely weak," a palace statement said.
Prince Rainier, 81, has been in hospital since 7 March and was taken into intensive care a week ago.
The palace announced on Thursday that Crown Prince Albert of Monaco had taken over his ailing father's duties.
Prince Rainier, who is being treated for heart, lung and kidney problems, came to the throne in 1949 and is Europe's longest-serving living monarch.
The palace statement said his medical team "in full agreement with the prince's children believe that it is right for the moment to continue active treatment".
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.