Pope Chairs 2013
Pope Benedict XVI. Reuters

This is a developing story. Stay tuned for updates.

Pope Benedict, the 265th Pope to lead the worldwide Catholic church, will step down on Feb. 28, according to a Vatican spokesman.

The 85-year-old pontiff cited his age in a brief statement as the reason for his decision, saying that "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 his role as the spiritual leader of 1.2 billion Catholics.

"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," the pope said, according to the Vatican. (Read the full resignation letter here.)

After Feb. 28, the cardinals will meet to elect a new pontiff, a decision which should not take too long, according to a Vatican spokesman. "Before Easter, we will have the new pope," the Rev. Federico Lombardi, said at a news conference.

Who will be the next pope? Speculation is already building as to Benedict's possible successor.

Pope Benedict appeared to have some health problems in the last year -- he started using a cane and recently seemed to have difficulty reading an address he delivered in Rome. The former Cardinal Joseph Ratzinger was one of the oldest new popes, at 78, when elected to succeed John Paul II in April 2005.

The pontiff has been considering the decision for months, in light of his medical difficulties. He was having increasing difficulty walking and was recently advised by his doctor not to take any more transatlantic trips, his brother, Georg Ratzinger, told Deutsche Presse-Agentur.

"His age is weighing on him," he said. "At this age my brother wants more rest."

The decision was particularly surprising since it is so rare for a pope to step down. The last pope to resign was Gregory XII in 1415, to end the Western Schism, a civil war in the Catholic church between several competitors.

At the Vatican, Pope Benedict's resignation came as a shock to some observers, NBC News said.