Speaking on Austrian radio, Cardinal Christoph Schoenborn said on Thursday that the pontiff was sick, handicapped and dying.
"The whole world is experiencing a pope who is sick, handicapped and dying. I don’t know how close to death he is, who is approaching the last days and months of his life," the cardinal said.
His remarks came on the same day the pope's private secretary sought to underplay concerns about the pontiff's failing health.
The pope, 83, and suffering from Parkinson's Disease cannot walk without help and has increasingly looked weaker than normal in recent public appearances.
He has also struggled to speak at times.
"The whole world is experiencing a pope who is sick, handicapped and dying"
Cardinal Christoph Schoenborn
Schoenborn, head of the Catholic Church in Austria and archbishop of Vienna, is seen as a possible successor to the pope, who celebrates the 25th anniversary of being head of the Roman Catholic Church later this month.
Earlier during the day, Archbiship Stanislaw Dziwisz, the pope's private secretary, said recent comments by another senior cardinal about the pope's condition had been taken out of context.
A German cardinal had earlier remarked that the pope was "in a bad way." He had called on the faithful to pray for the pope.
The Vatican has insisted that the pope intends to push ahead with a hectic programme of events this month, including the 25th anniversary celebrations, the beatification of Mother Teresa and a ceremony to invest some 30 new cardinals.