Pope Benedict XVI is to step down at the end of February as head of the Catholic Church, saying he is no longer strong enough to fulfil his duties.
The 85-year-old German-born pontiff made the announcement in Latin in a speech in the Apostolic Palace on Monday that he is resigning as leader of the world's 1.2 billion Catholics after nearly eight years.
His voice was barely audible as he read out the statement, saying he no longer has the mental and physical strength to cope with the demands of his ministry.
The Vatican said it expects that a new Pope would be elected before Easter - the first time in centuries that a current and former Pope are alive at the same time.
Cardinal Joseph Ratzinger became the head of the Church in 2005 following the death of John Paul II.
His brother Geog Ratzinger, from Germany, said Benedict had been advised by his doctor not to take any more trans-Atlantic trips and had been considering stepping down for months.
Mr Ratzinger said the was having increasing difficulty walking and that his resignation was part of a "natural process. His age is weighing on him. At this age, my brother wants more rest."
The conclave which will choose the next Pope is unlikely to elect someone radically at odds with Benedict. In his time at St Peter's, he has personally appointed many new cardinals who share his conservative theological and spiritual convictions, the BBC reports.
There is speculation that his successor could be from Latin America, home to more than 40% of the world's Catholics. Leading contenders include the Archbishop of Sao Paulo Odilo Scherer and Cardinal Leonardo Sandri from Argentina.
If the papacy went to an African, frontrunners would be Nigeria's Cardinal Francis Arinze or Ghana's Cardinal Peter Turkson.
Pope Benedict is not expected to take part in the conclave. After stepping down on 28 February he will retire to a monastery inside the historic walls of the Holy See and is likely to remain out of public view. While the monastery is renovated for him, he will stay at the papal summer house at Castel Gandolfo near Romem.
Benedict's is the first papal resignation in nearly 600 years. The last to step aside was Pope Gregory XII, who resigned in 1415 amid a schism within the Catholic Church. In 1294 Celestine V, a hermit elected to end a deadlock among cardinals, stepped down after reigning five months.
At 78, Cardinal Joseph Ratzinger was one of the oldest new popes in history when elected. The first German pope for about 1000 years, he travelled regularly, making about four foreign trips a year.
A theological conservative, he has uncompromising views on homosexuality and women priests. He took the helm as one of the fiercest storms the Catholic Church has faced in decades - the scandal of child sex abuse by priests.
Before his election, Cardinal Ratzinger spent 24 years in charge of Congregation for the Doctrine of the Faith.
The ABC reports the first major crisis of his pontificate arose from a university lecture in Germany when he quoted a 14th century Byzantine emperor as saying Islam had only brought evil to the world and that it was spread by the sword. The Pope later said he regretted any misunderstanding the speech caused.
In a move that was widely seen as conciliatory, in late 2006 he made a trip to Turkey and prayed in the Blue Mosque in Istanbul with a Turkish Mufti.
Sex abuse victims welcome resignation
Survivors of sexual abuse say Pope Benedict XVI has done nothing to punish paedophile priests or church seniors who looked the other way.
In the United States, a group representing 12,000 people who say they have been abused by priests is describing his legacy as one of shame. The survivors' network, known as SNAP, has members worldwide.
Joelle Casteix, an American spokesperson, told Radio New Zealand's Checkpoint programme on Tuesday her first reaction to the news of the Pope's resignation was one of shock, but now she sees that it makes sense.
"The past five years there have been child sex abuse inquiries against the Catholic Church across the globe. The sex abuse scandal has reached every level of the Catholic Church and his health has declined.
"So it seems perfectly reasonable that he should resign in order to step away from much of the criticism that he's been receiving."
The Irish Survivors of Child Abuse support group said Pope Benedict ultimately delivered nothing despite promising to address the decades of abuse.
In Australia, the Survivors Network of those Abused by Priests issued a statement saying the pontiff had done little to stop ''the reign of terror of child rapist priests''.
Leaders pay tribute
Political leaders have praised Pope Benedict and hailed his courage in resigning.
German Chancellor Angela Merkel described the Pope, who was born in Bavaria, of the greatest religious thinkers of our time.
Italy's Prime Minister Mario Monti said he was "greatly shaken" by the resignation, while French President Francois Hollande said the decision was eminently respectable.
British Prime Minister David Cameron said the Pope would be missed as a spiritual leader to millions, while the Archbishop of Canterbury, Justin Welby, said he held the office with great dignity.
United States President Barack Obama extended his appreciation and prayers to the Pope.
In Israel, the Chief Rabbi praised the Pope's ability to reach out to other faiths and wished him good health, but British Muslim group the Ramadhan Foundation condemned his comments on Islam in the early years of his papacy.
New Zealand Prime Minister John Key said the news of his resignation will come as a shock to Catholics around the world, but the Pope obviously feels his health won't allow him to carry out his duties effectively.