The 95-year-old former pontiff resigned from the position nine years ago.
“I want to ask you all for a special prayer for Pope Emeritus Benedict who sustains the Church in his silence. He is very sick,” Francis said.
“We ask the Lord to console and sustain him in this witness of love for the Church to the very end.”
A spokesperson for the Vatican confirmed to CNN that “in the last few hours there has been a deterioration due to the advancement of (Benedict’s) age.”
“The situation at the moment remains under control and continually monitored by his doctors,” the spokesman, Matteo Bruni, said.
Benedict is one of the few popes in history to resign from the position, as popes generally serve until their deaths. He resigned citing "advanced age." It was the first time a pope had resigned in nearly 600 years.