The possibility has been raised that Pope Benedict XVI should resign over the snowballing paedophile priest scandal in the Catholic Church. The BBC's Vatican correspondent, David Willey, examines the question.
The Vatican has vigorously defended Pope Benedict's record in office
In theory, there is nothing to stop Pope Benedict taking a piece of paper out of his writing desk and drafting a letter of resignation to hand to the College of Cardinals, the supreme electoral body of the Catholic Church.
Under Canon Law, the only conditions for the validity of such a resignation are that it be made freely and be properly published.
But no pope has done this in modern times.
There has, however, been persistent speculation by historians that during World War II, Pope Pius XII drew up a document stating that if he were to be kidnapped by the Nazis he was to be considered to have resigned, and a successor should be chosen.
As the Vatican has delayed the full release of its archives relating to Pius's pontificate, because of a dispute over his reaction to the Nazi Holocaust, there is no means of verifying whether this is true.
Going back further in time, the last case of a pope resigning dates back a further five centuries. Pope Gregory XII - who reigned from 1406 to 1415 - did so to end what was called the Western Schism.
No pope has resigned as head of the Catholic Church in modern times
There were three rival claimants to the papal throne at that time, the Roman Pope Gregory XII, the Avignon Pope Benedict XIII, and the Antipope John XXIII. Before resigning, Gregory formally convened a Church Council and authorised it to elect his successor.
The only other significant example of a papal resignation dates back even further in time.
In 1294, Pope Celestine V, only five months after his election, issued a solemn decree declaring it permissible for a pope to resign and then did so.
He lived for two further years as a hermit, and was later declared a saint. The decree that he issued ended any doubt among canon lawyers about the validity of a papal resignation.
Having said all this, the likelihood of Pope Benedict voluntarily laying down his high office remains slim.
The Vatican is for the moment vigorously defending the record of his five-year papacy, and Benedict's previous conduct during the period when he was Cardinal Archbishop of Munich and subsequently head of the CDF, the Congregation of the Doctrine of the Faith, the watchdog Vatican department responsible for disciplining priests guilty of bad conduct.