Pope Benedict XVI surprised the world and left the Catholic church stunned when he said on Monday that he would resign – the first pope to do so since the middle ages.
The move, announced without warning, will take place on 28 February and leave the papacy vacant until a successor is chosen.
A Vatican spokesman said Benedict's aides were "incredulous" when he told them he would step down because he was too weak to fulfil his duties. He summoned a meeting of cardinals to tell them of "a decision of great importance for the life of the church".
One of those called to hear the announcement, the Mexican prelate Monsignor Oscar Sanchez, said none of the cardinals had expected it. "The pope took a sheet of paper and read from it. He just said that he was resigning and that he would be finishing on February 28," he said.
"The cardinals were just looking at one another. Then the pope got to his feet, gave his benediction and left. It was so simple; the simplest thing imaginable. Extraordinary. Nobody expected it. Then we all left in silence. There was absolute silence … and sadness."
His successor is expected to be elected by the end of March and possibly for the beginning of holy week on 24 March. Benedict will honour public commitments and engagements until the date of his resignation, after which he will move to a summer residence near Rome and then to a former monastery within Vatican territory.
He will take no part in the process to elect a successor. Cardinals will meet and vote on nominees in a series of ballots until a new pope is chosen.
Federico Lombardi, a Vatican spokesman, said Benedict, 85, had resigned not because of "difficulties in the papacy" or a specific illness but instead a progressive decline in his strength.
"In the last few months he has seen a decline in vigour, both of the body and spirit," Lombardi told reporters. "It was his personal decision taken with full freedom, which deserves maximum respect."
In a statement, Benedict said: "After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry.
"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of bishop of Rome, successor of Saint Peter, entrusted to me by the cardinals on 19 April 2005, in such a way that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a conclave to elect the new supreme pontiff will have to be convoked by those whose competence it is."
Benedict, who became the 265th pope in 2005, has arthritis, particularly in his knees, hips and ankles. He had been due to travel to Brazil, the largest Catholic country in the world, in July for a youth festival, but concerns had been raised among Vatican observers about whether he was well enough.
A voluntary papal resignation is rare – certainly in recent centuries. Pope Celestine V exercised his right to abdicate in 1294.