Catholics are praying for Pope Emeritus Benedict XVI after Pope Francis revealed that he is "very ill".
Pope Francis asked the faithful to offer a "special prayer" for his predecessor during his General Audience on Wednesday.
"Remember him - he is very ill - asking the Lord to console him and to sustain him in this witness of love for the Church, until the end," he said.
The Holy See press office said that there had been an "aggravation" in Benedict's condition "due to advancing age", although specific details have not been disclosed.
In an update on Thursday, the Holy See press office said that Benedict's condition was serious but stable and that he remained alert.
"The Pope Emeritus managed to rest well last night, he is absolutely lucid and alert and today, although his condition remains serious, the situation at the moment is stable," said Matteo Bruni, Director of the Holy See Press Office.
Benedict, 95, resigned as Pope in 2013 and has since been living in a monastery within the Vatican where he remains under round-the-clock supervision by doctors.
He was visited by Pope Francis on Wednesday.
Responding to the appeal for prayer, Cardinal Vincent Nichols, head of the Catholic Church in England and Wales said, "Let us join with Pope Francis in praying for Pope Emeritus Benedict, and asking the Lord to sustain and console him at this time."
Speaking on LBC Radio on Thursday, the Cardinal praised Benedict's "outstanding strength and ability" as a thinker and theologian and said that while pope, he had wanted "a healthy dialogue" between "the world of reason and the world of faith".
Commenting on Benedict's two volumes on Jesus, Cardinal Nichols continued, "They tackled some of the most complex scriptural challenges in the most straightforward of ways and opened up in a very scholarly but simple way the life of Jesus that is absolutely central to Christianity."
The Archbishop of New York, Cardinal Timothy Dolan, said in a statement, "With great faith in God's goodness, I join with Pope Francis in praying for Pope Emeritus Benedict XVI, and ask the Catholic faithful of the Archdiocese of New York, and all people of good will go do the same."
The Archbishop of San Francisco, Salvatore Cordileone, called Benedict "a great soul and a great inspiration to millions" as he asked people to join him in praying for him.
