Pope Benedict XVI is to resign as head of the Catholic Church on February 28, The Vatican has revealed.
Pope Benedict said in a statement that his strength is no longer adequate to continue in office due to old age, according to Reuters.
He said he was "fully aware of the gravity of this gesture".
The 85-year-old said he had noticed that his strength had deteriorated over recent months "to the extent that I have had to recognize my incapacity to adequately fulfil the ministry entrusted to me".
"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter."
He becomes the first pontiff to resign in nearly 600 years, announcing his decision in Latin during a meeting of Vatican cardinals on Monday morning.
A Vatican official said the period between Pope Benedict's resignation and the election of his successor will be "as brief as possible."
The Pontiff's spokesperson, Federico Lombardi, said Pope Benedict's decision was 'in full compliance with church law', and that he wasn't resigning due to any 'difficulties in the papacy'.
But Pope Benedict's decision to step down was seen as all the more unusual given that most incumbents die in office.
The decision sets the stage for a conclave to elect a new pope before the end of March.
There are several papal contenders in the wings, but no obvious front-runner as was the case when Pope Benedict was elected pontiff in 2005 after the death of Pope John Paul II.
The Pope decided to resign "after much examination of conscience and much reflection", according to the Archbishop of Westminster.
As the shock decision emerged on Monday morning, tributes were paid to Pope Benedict and the contribution he made to Roman Catholicism.
[Read more: Full text of his declaration]
The three most senior figures in the Catholic Church in England and Wales have paid tribute to Pope Benedict XVI after the Pontiff announced he was leaving office on grounds of age and infirmity.
The Most Rev Vincent Nichols, Archbishop of Westminster and leader of Catholics in England and Wales, called on "people of faith" to pray for the 85-year-old Pontiff, saying that his announcement had shown "great courage."
Related: Pope Benedict XVI resigns: Live blog
"Pope Benedict's announcement today has shocked and surprised everyone," he said in a statement.
"Yet, on reflection, I am sure that many will recognise it to be a decision of great courage and characteristic clarity of mind and action.
Here is the Pope's last tweet before today's announcement, sent yesterday.
The Archbishop of York, John Sentamu, was one of many who reacted to the news on Twitter.
Cardinal Cormac Murphy-O'Connor, who retired as Archbishop of Westminster in 2009, said: "My reaction was one of surprise and then gratitude for his service and leadership of the Church over the past seven years in troubled times.
"He has been a great teacher, thinking particularly of his visit to Britain and the example he gave of being a Good Shepherd and a good pastor.
The Most Rev Peter Smith, Archbishop of Southwark, said he had been "quite taken aback" by the announcement.
"I was quite taken aback, because it came as a complete surprise and completely out of the blue," he said.
He added that he was still trying to "digest" the news having been in a meeting this morning.
"My first thought when I heard the news that he was resigning, my instinct was that it is because of his health and his frailty and he feels it is an incredibly responsible task to be the chief shepherd of the Church on earth," he said.
"I think it shows great wisdom, sensitivity and humility."
The two most senior figures in the Church of England also paid tribute to the pope.
The Most Rev Justin Welby, the new Archbishop of Canterbury, said: "It was with a heavy heart but complete understanding that we learned this morning of Pope Benedict's declaration of his decision to lay down the burden of ministry as Bishop of Rome, an office which he has held with great dignity, insight and courage.
"As I prepare to take up office I speak not only for myself, and my predecessors as archbishop, but for Anglicans around the world, in giving thanks to God for a priestly life utterly dedicated, in word and deed, in prayer and in costly service, to following Christ.
"He has laid before us something of the meaning of the Petrine ministry of building up the people of God to full maturity."
Pope Benedict's decision to retire should not come as too much of a surprise.
He said after he was elected to the Papacy that he had prayed not to get the post and was hoping for a peaceful old age.
As the powerful Cardinal Joseph Ratzinger, he was already well-known within the Catholic world before his election to the top job.
His image on elevation to the Papacy was one of an enforcer of Catholic orthodoxy and a cerebral disciplinarian who was unafraid to crack down on liberals and dissidents within the church.
While Prefect of the Congregation for the Doctrine of the Faith (CDF), he gained the nickname "God's Rottweiler" for his pursuit of Catholic theologians and clergy seen to stray from orthodox teaching.
Pope Benedict XVI was born Joseph Ratzinger in 1927 and was the 265th Pope.
Ratzinger was 78 when he was elected to become Pope in April 2005 and succeeded John Paul II.