ROME – Pope Emeritus Benedict XVI is in serious but stable condition after Pope Francis asked for prayers for his "very ill" predecessor whose respiratory problems have worsened lately, Vatican sources said Thursday.

Sources in contact with the former monastery where the 95-year-old ex-pontiff lives told ANSA that nothing had changed with respect to Wednesday, when Francis issued his appeal at his general audience.

They said the former Joseph Ratzinger, the first German pope in 1,000 years, spent the night under the constant control of his doctors and their monitoring of his condition would continue unabated.

Pope Francis on Wednesday asked the faithful to pray for his predecessor, who he said was "very sick".

Speaking at the end of his weekly general audience in the Vatican, Francis asked for a "special prayer" for the German ex-pontiff, who became the first pope to resign in 600 years when he stepped down due to ill health on Feb. 11, 2013.

The man born Joseph Ratzinger, who had taken over from Polish pope Saint John Paul II on April 19, 2005, is reportedly suffering from increasing respiratory problems.

His last published photos, on the occasion of a prize giving for the two winners of his Ratzinger Foundation theology prize on Dec. 1, show a man slumped in his chair, visibly thinner, weak and very fatigued.

"I would like to ask all of you for a special prayer for Pope Emeritus Benedict who in silence is supporting the Church," Francis said after his audience in the Vatican. "Remembering him, he is very ill, asking the Lord to console him, to sustain him in this witness of love for the Church, until the end".

The 86-year-old Francis is himself not in the best of health and has often been forced to use a wheelchair due to chronic knee pain. On Wednesday he used a stick to get around during the audience, before heading off to visit Benedict.

Vatican spokesman Matteo Bruni subsequently said: "Regarding the health conditions of the Pope Emeritus, for whom Pope Francis asked for prayers at the end of the general audience this morning, I can confirm that in recent hours there has been an aggravation due to advancing age.

"The situation at the moment remains under control, followed constantly by the doctors. At the end of the general audience Pope Francis went to the Mater Ecclesiae monastery to visit Benedict XVI. We join with him in prayer for the Pope Emeritus".

Benedict's condition had already worsened in the days before Christmas, well-informed sources told ANSA later Wednesday, when "he began to experience respiratory problems".

Benedict XVI spent Christmas at the former Mater Ecclesiae monastery where a personal Mass for him was celebrated at home in the little chapel, the sources said, adding that the former pope was getting treatment.

Mathematician and atheism campaigner Piergiorgio Odifreddi said after a recent meeting with the pope emeritus that he was "by now completely unable to speak".

The bombshell news that Benedict was to become the first pope to resign since Gregory XII in 1415, and the first to do so on his own initiative since Celestine V in 1294, was broken by ANSA Vaticanist Giovanna Chirri at 10:46 on Feb. 11, 2013 thanks to her knowledge of Latin, the language in which the pope had cited poor health for his reasons in quitting, unbeknownst to the Latin-less rest of the Vatican press corps.

Once dubbed “God's Rottweiler” in his previous role as the Vatican's doctrinal enforcer, the meek intellectual and cerebral theologian Benedict struggled to keep a firm hand on the papacy and was beset with clerical sex abuse scandals, upping action against them.

He was recently himself accused of inaction on predator priests as a bishop in his native Germany. (ANSA)