Pope Benedict XVI is to resign on February 28, the Vatican has announced.
He will be the first head of the Catholic Church to resign in almost 600 years. The Vatican said his departure would leave the post temporarily vacant.
Vatican spokesman Federico Lombardi said: "The pope announced that he will leave his ministry at 8:00pm (7pm UK time) on February 28."
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."
Edward Pentin, from the Catholic Herald, told Sky News: "It really is too early to say what his greatest achievement has been."