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.
His papacy has been beset by a child sexual abuse crisis that tarnished the Church, one address in which he upset Muslims and a scandal over the leaking of his private papers by his personal butler.
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 recognize my incapacity to adequately fulfill 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 said according to a statement from the Vatican.
A Vatican spokesman said the pontiff would step down from 2 p.m. ET 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.
Elected to the papacy on April 19, 2005 when he was 78 -- 20 years older than John Paul was when he was elected -- he ruled over a slower-paced, more cerebral and less impulsive Vatican.
But while conservatives cheered him for trying to reaffirm traditional Catholic identity, his critics accused him of turning back the clock on reforms by nearly half a century and hurting dialogue with Muslims, Jews and other Christians.
Before he was elected Pope, the former Cardinal Joseph Ratzinger was known by such critical epithets as "God's rottweiler" because of his stern stand on theological issues.
ROME (Reuters) - The following is the text of the speech that Pope Benedict delivered in Latin to members of a consistory, or church council, announcing his planned resignation on Monday. It was distributed by the Vatican in a statement:
I have convoked you to this Consistory, not only for the three canonizations, but also to communicate to you a decision of great importance for the life of the Church. After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry. I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering. However, in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, 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 recognize my incapacity to adequately fulfill 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, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.
"Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects. And now, let us entrust the Holy Church to the care of Our Supreme Pastor, Our Lord Jesus Christ, and implore his holy Mother Mary, so that she may assist the Cardinal Fathers with her maternal solicitude, in electing a new Supreme Pontiff. With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer."
(Reporting by Steve Scherer)