Pope Benedict surprised the world on Monday by saying he no longer had the mental and physical strength to cope with the demands of his ministry, becoming the first pontiff to step down since the Middle Ages and leaving his aides "incredulous".
A Vatican spokesman said the pontiff would step down from 1900 GMT on February 28, leaving the office vacant until a successor was chosen to Benedict who succeeded John Paul, one of history's most popular pontiffs.
The 85-year-old German-born Pope, hailed as a hero by conservative Catholics and viewed with suspicion by liberals, said he had noticed that his strength had deteriorated over recent months.
A Vatican spokesman said the Pope had not resigned because of "difficulties in the papacy" and the decision had been a surprise, indicating that even his closest aides were unaware that he was about to quit. The Pope does not fear schism in the Church after his resignation, the spokesman said.
In a statement, the pope said in order to govern "...both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognise 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."
The Vatican stressed on Monday that no specific medical condition prompted Benedict's decision to become the first pontiff to resign in 600 years. Still, Benedict said his advanced age means he no longer has the necessary mental and physical strength to lead the world's more than one billion Roman Catholics.
A doctor familiar with the pope's medical team told The Associated Press on Monday that the pontiff has no grave or life-threatening illnesses. But, the doctor said, the pope - like many men his age - has suffered some prostate problems. Beyond that, the pope is simply old and tired, the doctor said on condition of anonymity.
"In someone who's 85 and has arthritis, the activities of being a pope will be a struggle," said Dr. Alan Silman, the medical director of Arthritis Research UK. He said Pope Benedict most likely has osteoarthritis, which causes people to lose the cartilage at the end of their joints, making it difficult to move around without pain.