Pope Benedict XVI is to resign on February 28, the Vatican has announced.
The 85-year-old's resignation letter 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.
"I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering.
"However, 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."
He will step down after almost eight years in the post, having been elected in April 2005.
The final post on his Twitter page, sent on Sunday, said: "We must trust in the mighty power of God's mercy. We are all sinners, but His grace transforms us and makes us new."
May God lead His Church in finding a new shepherd even as he rests the old.
Come Holy Spirit, come.