The Holy See said Benedict was receiving constant medical care.
“I would like to ask all of you for a special prayer for Pope Emeritus Benedict, who, in silence, is sustaining the church," Francis said at the end of his general audience earlier Wednesday. "He is very sick, asking the Lord to console and sustain him in this witness of love for the church, until the end.”
Vatican spokesman Matteo Bruni said later: “I can confirm that in the last few hours, his condition worsened due to age. The situation at present is under control, monitored constantly by doctors.”
Benedict, 95, in 2013 became the first pope in more than 600 years to resign, citing his advanced age and ebbing strength amid the clerical sexual abuse scandal that engulfed the church. He has been living in the Vatican since then.
Formerly Cardinal Joseph Ratzinger of Germany, Benedict was born in Marktl, Bavaria, in 1927. He was drafted into the German anti-aircraft corps at 16 and was briefly detained by U.S. troops in 1945 before he was released back to the seminary.
Ratzinger was ordained in 1951, rising to become the head of the Catholic Church in 2005 at age 78.
An independent report in January found that he had failed to act in four cases of sexual assault when he was the archbishop of Munich from 1977 to 1982. He apologized for the handling of the cases but admitted no wrongdoing.
Benedict has largely stayed out of the public eye since his retirement.
Claudio Lavanga reported from Rome and Rhoda Kwan reported from Taipei, Taiwan.