Pope Francis on Wednesday asked for prayers for former pope Benedict, saying he is "very sick".
The Pope made the surprise appeal at the end of his general audience, but provided no further details.
"I would like to ask all of you for a special prayer for Pope Emeritus Benedict, who, in silence, is sustaining the Church," Pope Francis said, speaking in Italian.
"Let us remember him. He is very sick, asking the Lord to console and sustain him in this witness of love for the Church, until the end."
The Vatican confirmed that the health of the former pontiff had worsened in the past few hours and that he had been visited by Pope Francis.
"I can confirm that in the last few hours there has been a deterioration due to advancing age. The situation at the moment remains under control, monitored continually by doctors," spokesman Matteo Bruni said in a statement.
In 2013, Benedict, 95, became the first pope in about 600 years to resign. The German pope emeritus, whose real name is Joseph Ratzinger, has been living in the Vatican since then.
His resignation created an unprecedented situation in which two "men in white" — Benedict and his successor, Pope Francis — co-existed within the walls of the tiny city state.
In 2020, Maltese Cardinal Mario Grech said Benedict "has difficulty in expressing himself".
The former pope, who uses a wheelchair, said "the Lord has taken away my speech in order to let me appreciate silence", Cardinal Grech told Vatican News.
In April, Benedict's long-time secretary, Archbishop Georg Gaenswein, told Vatican News the former pope was "physically relatively weak and fragile", but "in good spirits".
Pope Benedict was 78 when he succeeded the long-reigning and popular John Paul II in April 2005.
His papacy was beset by Church infighting and an outcry over a sex abuse scandal and international investigation, which rocked Catholic churches throughout the globe.
He became the first pontiff to apologise for the abuse scandals, expressing "deep remorse" and meeting with victims in person.
But while he took key steps to tackling child abuse by members of the clergy, he was criticised for failing to end Church cover-ups.
The scandal has returned to haunt him in retirement.
A damning report for the Catholic Church in Germany in January 2022 accused him of personally failing to stop four predatory priests in the 1980s while archbishop of Munich.
Benedict has denied wrongdoing and the Vatican has strongly defended his record.
Unlike his successor, Pope Francis, a Jesuit who delights in being among his flock, Benedict is considered a conservative intellectual.
He was described as "God's Rottweiler" in a previous post.
But as pontiff he appeared overwhelmed by the challenges facing a Church that was losing influence and followers, and years of Vatican turmoil took their toll.
He stepped down in February 2013 in an announcement delivered to cardinals in Latin, later saying the decision was the result of a mystical experience.
Pope Francis, 86, has said he may also resign at some point.
He revealed for the first time in an interview this month that he signed a resignation letter nearly a decade ago should poor health prevent him from carrying out his duties.