Pope Benedict XVI had been agonising over whether to step down for months, his brother has revealed.
The Pope's brother, Georg Ratzinger, said the pontiff had been advised by his doctor not to take any more transatlantic trips.
Mr 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."
The Pope told Vatican cardinals this morning that being the leader of more than a billion Roman Catholics worldwide requires "both strength of mind and body".
Benedict himself raised the possibility of resigning if he were simply too old or sick to continue on in 2010, when he was interviewed for the book "Light of the World."
"If a pope clearly realises that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign," Benedict said.
The former Cardinal Joseph Ratzinger had an intimate view as Pope John Paul II, with whom he had worked closely for nearly a quarter-century, suffered through the debilitating end of his papacy.
But The Vatican had 'no warning' that the Pope was about to resign.
Vatican spokesman Federico Lombardi said the Pope's decision to step down "took us by surprise" and had left his aides "incredulous."
Speaking at a hastily-arranged press conference, Mr Lombardi said: "The Pope caught us a bit by surprise."
He insisted the Pope was not resigning because of "difficulties in the Papacy" and added Benedict did not fear a divide in the Church after his resignation.