But Vatican spokesman Federico Lombardi reiterated that he was not stepping down because of any specific illness.
His last public appearance will be his final mass in Saint Peter's Square on 27 February, Fr Lombardi said.
The pontiff would have no role in the running of the church after his resignation, he added.
The unexpected development - the first papal resignation in nearly 600 years - surprised governments, Vatican-watchers and even the Pope's closest aides.
Cardinal Joseph Ratzinger became Pope Benedict XVI in 2005 after John Paul II's death.
The BBC's David Willey in Rome says that in theory there has never been anything stopping Pope Benedict or any of his predecessors from stepping aside.
Under the Catholic Church's governing code,
Canon Law, the only conditions for the validity of such a resignation
are that it be made freely and be properly published.
But resignation is extremely rare: the last pontiff to step
aside was Pope Gregory XII, who resigned in 1415 amid a schism within
the Church.'Diminishing strength' According to a report in Italy's Il Sole 24 newspaper, the Pope had surgery to replace a pacemaker just under three months ago.
At a news conference at the Vatican, Father Lombardi confirmed that the batteries in the pacemaker, which had been fitted several years ago, had been replaced in the routine operation.
