Vatican spokesman, Federico Lombardi said: "The Pope announced that he will leave his ministry at 8:00 pm (1900 GMT) on February 28."

He said that the Pope was not stepping down because of any specific illness. Depression or uncertainty were ruled out as causes for the resignation.

The announcement was "a bolt from the blue", said Angelo Sodano, a senior Vatican cardinal. The news apparently came as a surprise, even to the Pope's close aides.

Greg Burke, another Vatican spokesman, told The Daily Telegraph there would be a new Pope by Easter.

"We can expect a new Pope by Easter, which this year falls on March 31," he said.

"Because the Pope has not died there is no need for the traditional nine days of mourning, but there will be a Conclave (a meeting of Cardinals to select the new Pope)."

The pope's brother Georg Ratzinger, reportedly said the pontiff had been advised by his doctor not to take any more transatlantic trips and had been considering stepping down for months.

Talking from his home in Regensburg to the news agency dpa, Georg Ratzinger said his brother was having increasing difficulty walking and that his resignation was part of a "natural process."

"His age is weighing on him," the 89-year-old said of his 85-year-old brother. "At this age my brother wants more rest."

Georg Ratzinger did not answer his telephone for calls seeking further comment.

The leader of the Catholic Church in England and Wales, the Archbishop of Westminster, the Most Rev Vincent Nichols said he had been shocked and surprised by the Pope's decision.

But he said: "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.

"The Holy Father recognises the challenges facing the Church and that strength of mind and body are necessary for his tasks of governing the Church and proclaiming the Gospel.

"I salute his courage and his decision. I ask people of faith to keep Pope Benedict in their prayers.

"We Catholics will do so, with great affection and the highest esteem for his ministry as our Holy Father remembering with joy his Visit to the United Kingdom in 2010.

"Pray, too, for the Church and all the steps that must take place in the next weeks. We entrust ourselves to the loving Providence of God and the guidance of the Holy Spirit."

The Archbishop of York released the following statement: "With the news that Pope Benedict XVI will resign at the end of February, the Christian world will miss a great theologian with great spiritual depth.

"We should remember Pope Benedict communicated the revelation of God in a characteristic way as a true successor of St Peter. He was unafraid to proclaim the Gospel and challenge a culture that is so self-referential, managing to lift our eyes to God's glory. "Let us all lift our eyes to the glory of God, embracing his love and care for this world."

Italy's prime minister Mario Monti tweeted: "I am very shocked by this unexpected news."

The pontiff steered the Catholic church for eight years after being chosen to succeed John Paul II in April 2005.

Since reportedly suffering a stroke in 1991, Benedict has suffered a series of health scares. In November 2006, it was reported that he had a routine heart examination. In July 2009, he was hospitalised after falling and breaking his wrist in the Alps.

Cardinal Philippe Barbarin of France revealed that the Pope has long suffered a heart condition and has been taking medication since his initial stroke.

Early in his pontificate, Benedict himself predicted that he would have a short papacy because of his health.

The last time a pope resigned was in 1415. Pope Gregory XII did so to end the Western Schism, at a time when there were three claimants to the papacy.

Before resigning, Gregory convened a council and authorised it to elect his successor.

A spokesman for Chancellor Angela Merkel said the German-born Pope deserved "respect" and "gratitude" for his nearly eight years as pontiff after he announced he was to step down.

"The federal government has the greatest possible respect for the Holy Father, for his accomplishments, for his life-long work for the Catholic Church," said Steffen Seibert, adding he deserved also "gratitude."

Last year the Pope started using a cane on occasions and recently he appeared to have trouble reading the text of an address he delivered in Rome.

In November last year, he declared that it is "beautiful to be old".

"The quality of a society, I'd say of a civilisation, is judged by how well it treats its elderly," the 85-year-old pontiff said during the visit to the home run by the Saint Egidio Community, a Catholic charity.

While it was "beautiful to be old", age brought challenges and difficulties with which he himself was familiar.

"I know well the difficulties, the problems and the limits of this age, and I know that these difficulties are aggravated for many people by the economic crisis," the Pope said.