In a break with tradition, Pope Benedict XVI, 85, says he plans to step down at the end of this month because of his deteriorating physical strength.
It is the first time this has happened in the Roman Catholic Church since 1415 and is likely to pave the way for a new pontiff by Easter.
In a speech in Latin to cardinals, the German-born Benedict, who has been in office since April 2005, says leading the world's 1.2 billion Catholics is a job that requires strength of both mind and body.
But he says his strength has "deteriorated in me to the extent that I have had to recognise my incapacity to adequately fulfill the ministry entrusted to me."
Reporters in Rome were told at a briefing that Pope Benedict had been thinking about the move for some time and was due to an illness.
Vatican spokesman Fr Frederico Lombardi says the pope will retire to a life of prayer and writing.
The resignation takes effect at 8pm on February 28 and will immediately be followed by a conclave, a gathering of cardinals, who will elect the new pope.
As there is no traditional nine-day mourning period, the selection can start straight away.
"This means we'll have a new pope by Easter," Vatican media adviser Greg Burke says.
Good Friday this year is on March 29.
Background and legacy
Born on April 16, 1927, in Bavaria, Germany, Joseph Ratzinger was the son of a policeman and became a priest and university lecturer.
He was named an archbishop in Munich in 1977 and was made a cardinal in the same year.
He was once known as Pope John Paul II’s “rottweiler” for his role as the Prefect of the Congregation of the Doctrine of the Faith.
His quick election as Pope at the age of 78 reflected his standing as one of the church’s leading intellectuals and theologians.
But his papacy has been marked by controversies of a less cerebral nature. The biggest was the wave of allegations that priests around the world sexually abused children.
Pope Benedict condemned the abuse and issued a raft of measures aimed at preventing any future abuses and making it easier to discipline and defrock priests.
In 2006, he sparked anger across the Muslim world for a controversial speech in which he cited a 15th-century Byzantine emperor's description of the Prophet Muhammad's teachings as "evil and inhuman."
And over the past year, his administration has been swept up in a leaked-documents controversy that exposed infighting among cardinals.
The Daily Telegraph’s Damian Thompson has this tribute:
Benedict XVI's achievements as pontiff have been remarkable. He has renewed the worship of the Church, reconnecting it to the majesty and deep piety of the past. He has forged new links with non-Catholics, for example by bringing ex-Anglicans into the fold through the Ordinariate.
He has promulgated teaching documents further integrating the love and teaching of Christ with the structures of the Church – structures that, it would appear, he feels now unable to continue ruling.
There have been been public relations disasters, notably over the readmission of ultra-traditionalist bishops to the Church, one of whom had Nazi sympathies. But there have been unexpected successes too: not least his remarkable visit to Britain, when his gentle wisdom profoundly touched even sceptics.