STORY: Ninety-five-year-old former pope Benedict, who in 2013 became the first pontiff in 600 years to step down, is "very sick".
That's according to his successor Pope Francis, who asked the church to pray for him on Wednesday (December 28).
"I would like to ask all of you for a special prayer for Pope Emeritus Benedict, who, in silence, is sustaining the Church.
"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 later said in a statement Benedict had suffered a sudden "worsening" of his health in the previous hours.
It also said his condition was "under control" and that he was receiving constant medical attention.
Francis, who visited the former pontiff shortly after his general audience, has often praised Benedict, likening him to a grandfather.
But the presence of two men dressed in white at the Vatican has at times been troublesome.
Conservatives have looked to the former pope as their standard-bearer.
Until a few weeks ago, those who'd seen Benedict said his body was very frail but his mind was still sharp.
These are the latest known photographs of Benedict.
They were taken on December 1st, when he met the winners of a prize for theologians named after him.