Benedict XVI's papacy will end on Feb. 28, the Pope himself announced on Monday.
The first pontiff to retire in six centuries, Benedict XVI made the shocking announcement in Latin during an "ecclesiastical council discussing the canonization of the martyrs of Otranto," the Italian news agency ANSA reported.
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The 85-year-old Pope cited his ingravescentem aetatem, or advanced age, as one of the reasons for his decision.
"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," the Pope said.
Benedict XVI admitted that his strength deteriorated "in the last few months."
"Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects," the Pope said.
The announcement was received by the Cardinals "with an astonished silence and a sense of loss," ANSA reported.
"It was a bolt out of the blue," Italian cardinal Angelo Sodano, the dean of the College of Cardinals, said.
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The number popes who have resigned has been estimated as high as 10, but the evidence is limited. Most recently (although it was hardly recent at all) Gregory XII resigned at the end of the Western Schism and a new pope was elected in 1417.
Born on April 16, 1927, in Marktl am Inn, Germany, as Joseph Ratzinger, Benedict XVI is said to suffer from joint pain and rheumatism.
Last year he started using a cane and recently appeared to have trouble reading the text of an address he delivered in Rome.
The Pope is also suffering from chronic atrial fibrillation, but refuses to take his prescribed anticoagulant medication, his doctors said.
A top-secret, leaked Vatican document, published last year by the Italian daily Il Fatto Quotidiano, predicted the Pope's death by November 2012.
The anonymous report revealed that Benedict XVI was secretly working on his succession, having chosen Milan cardinal Angelo Scola as the next Pope.
Indeed, cardinal Scola is regarded by global Vatican watchers as the favorite to succeed Benedict.
Cardinal Peter Turkson of Ghana and Cardinal Oscar Rodriguez Maradiaga of Honduras are also in the frame.
The papacy will be vacant as of Feb. 28, 2013, at 8 pm Italian time.
"A Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is," Benedict XVI told the Cardinals.
Image: Pope Benedictus XVI during the general audience in S. Peter, Vatican City on the 26th March, 2008. Credit: Broc/Wikimedia Commons.