Pope Emeritus Benedict XVI is “very ill" and being monitored by doctors, the Vatican confirmed on Wednesday.
His successor Pope Francis has asked the faithful to pray for the retired pontiff so God will comfort him “to the very end.”
Francis didn't elaborate on the condition of Benedict, who was the first pope to retire in 600 years in February 2013.
The Pope went to visit his predecessor in the monastery on Vatican grounds where Benedict has lived since stepping down.
The 95-year-old has become increasingly frail in recent years as he dedicated his post-papacy life to prayer and meditation.
On Thursday, the Vatican reported that Pope Emeritus was "able to rest well" and is "absolutely lucid and alert".
"Although his condition remains serious, the situation remains stable at the moment," a spokesperson added.
"Regarding the health conditions of the emeritus pope, for whom Pope Francis asked for prayers at the end of his general audience this morning, I can confirm that in the last hours, a worsening due to advanced age has happened," Vatican spokesperson Matteo Bruni said in a written statement on Wednesday.
"The situation at the moment remains under control, constantly monitored by doctors," Bruni added.
At the end of his customary Wednesday audience, Francis departed from his prepared remarks to say that Benedict is "very sick" and asked the faithful to pray for the retired pontiff.
"I would like to ask you all for a special prayer for Pope Emeritus Benedict, who is sustaining the church in silence,” Francis said.
"Remember him — he is very ill — asking the Lord to console him and to sustain him in this testimony to love for the church, until the end."
In Benedict’s native Germany, the head of that nation’s bishops’ conference, Limburg Bishop Georg Baetzing, joined in Francis’ call for prayers.
“My thoughts are with the emeritus pope,” Baetzing told the DPA news agency. “I call on the faithful in Germany to pray for Benedict XVI.”
A spokesperson for German Chancellor Olaf Scholz also said that his "thoughts" were with Benedict.