Pope Francis has announced the former Pope Benedict XVI, is 'very sick' and called for prayers for his predecessor.
The Pontiff surprised followers with the appeal as he finished his general audience at the Vatican, providing no further details.
Pope Benedict XVI, now 95, was the head of the Catholic Church between April 2005 and February 2013.
He resigned from the post aged 85 in 2013, making him the first to quit in 600 years. He resigned from the Church's top spot, saying his old age had limited his abilities to carry out his sacred duties.
He now lives in a monastery on the grounds of the Vatican.
Benedict's predecessor has pledged to a similar plan for his future by having submitted a resignation letter after his 2013 election.
The note - which was shared with the Secretary of Vatican City State - would come into affect should he become ill.
Francis - who is largely in good health - has recently struggled with knee pain that has left him needing to use a wheelchair.
But during public appearances, he uses a cane and has played down the ailment in interviews.
Speaking with Spanish outlet ABC, he said: "One governs with the head not the knee."
When asked what happens when a Pope is unable to fulfill their duties due to health issues, he answered: "In practice there is already a rule.
"I have already signed my renunciation."
It comes days after the pope used his Christmas message to lament the "icy winds of war" buffeting humanity and to make an impassioned plea for an immediate end to the fighting in Ukraine, a 10-month-old conflict he described as senseless.
At noon local time, Francis delivered the traditional Urbi et Orbi (Latin for to the city and to the world) speech from the central balcony of St Peter's Basilica.
Tens of thousands of tourists, pilgrims and residents of Rome crowded into St Peter's Square to listen to the pontiff and to receive his blessing.
Francis also mentioned long-running conflicts in the Middle East, including in the Holy Land, "where in recent months violence and confrontations have increased, bringing death and injury in their wake".
He also prayed for a lasting truce in Yemen and for reconciliation in Iran and Myanmar.
He lamented that at Christmas, the "path of peace" is blocked by social forces that include "attachment to power and money, pride, hypocrisy, falsehood".
"Indeed, we must acknowledge with sorrow that, even as the Prince of Peace is given to us, the icy winds of war continue to buffet humanity," Francis said.
"If we want it to be Christmas, the birth of Jesus and of peace, let us look to Bethlehem and contemplate the face of the child who is born for us," he said.
"And in that small and innocent face, let us see the faces of all those children who, everywhere in the world, long for peace."
