Pope Benedict XVI has announced that he is to resign at the end of the month.
The 85-year-old Pope says he is too old to continue in his role as leader of the Roman Catholic Church.
He was already one of the oldest new popes when he succeeded the late John Paul II in 2005 at the age of 78.
The Pope explained his unexpected decision in a statement.
"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," he said.
"In today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognise my incapacity to adequately fulfil the ministry entrusted to me."
The Pope steps down on 28 February, becoming the first to do so since Pope Gregory XII stepped down in 1415.
The news has been met by widespread surprise. Writing in The Telegraph, Damian Thompson said Catholics would be "deeply shocked and, in most cases, dismayed by this decision".
Despite this, he said he saw it as evidence of Pope Benedict's "deep humility".
"[This is] an act of self-sacrifice by a man not prepared to see the Church suffer as a result of his increasing frailty," he said.
News of Pope Benedict's resignation is trending on Twitter, with 24-7Prayer tweeting: "Praying peace, wisdom, strength, faith, hope for the Catholic Church today."
