(CNN) -- Pope Benedict XVI is considering changing the Vatican constitution to allow a vote for his successor to begin before March 15, Vatican spokesman the Rev. Federico Lombardi said Wednesday. Benedict, 85, shocked the world last week when he announced his intention to stand down effective February 28, citing the frailty of old age. Existing rules say the Roman Catholic Church's cardinals should start voting on a replacement from 15 to 20 days after the papal throne becomes vacant. With Benedict's resignation due to start February 28, the cardinals' conclave ordinarily would start no sooner than March 15. But Lombardi has said that because Benedict was leaving the papacy through resignation rather than death, the Vatican would explore the possibility of selecting a new pope sooner than normally prescribed.
Existing rules say vote for replacement should begin 15 to 20 days after papal vacancy . But Vatican has said because Benedict XVI is resigning, a vote might be held sooner than usual . Benedict's resignation is set for February 28 .