Pope Benedict XVI
Pope Benedict XVI will not interfere in choosing his successor after his shock decision to resign at the end of the month, the pontiff's brother has said.
Georg Ratzinger told the BBC the Pope would only "make himself available" if he were needed.
Benedict said on Monday he would resign after nearly eight years as the head of the Catholic Church because he was too old to continue at the age of 85.
The Vatican now says it expects a new Pope to be elected before Easter.
The unexpected development - the first papal resignation in nearly 600 years - surprised governments, Vatican-watchers and even Benedict'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 Pope to step aside was Pope Gregory XII, who resigned in 1415 amid a schism within the Church.
Speaking to the BBC from his home in Regensburg, Germany, Georg Ratzinger said his brother had been advised by his doctor not to take any more transatlantic trips and had been considering stepping down for months.
"When he got to the second half of his 80s, he felt that his age was showing and that he was gradually losing the abilities he may have had and that it takes to fulfil this office properly."
He said the resignation therefore was part of a "natural process".
And he added: "Where he's needed he will make himself available, but he will not want to want to intervene in the affairs of his successor."
The next Pope will be chosen by members of a 117-strong conclave held in the Sistine Chapel at the Vatican.
Analysts say Europeans are still among the favourites, including the current Archbishop of Milan, Angelo Scola, and Christoph Schoenbron - a former Austrian student of Benedict.
But strong candidates could emerge from Africa and Latin America, which both have very large Catholic populations. Among the names being mentioned are Ghana's Cardinal Peter Turkson and Cardinal Francis Arinze of Nigeria.
The Pope was to retire to the papal residence at Castel Gandolfo when he leaves office, the Vatican said, before moving into a renovated monastery used by cloistered nuns for "a period of prayer and reflection".
At 78, Cardinal Joseph Ratzinger was one of the oldest popes in history at his election.
He took the helm as one of the fiercest storms the Catholic Church has faced in decades - the scandal of child sex abuse by priests - was breaking.
The pontiff said in his Monday's statement: "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."
A theological conservative before and during his time as Pope, he has taken traditional positions on homosexuality and women priests, while urging abstinence and continuing opposition to the use of contraceptives.
His attempts at inter-faith relations were mixed, with Muslims, Jews and Protestants all taking offence at various times, despite his efforts to reach out and make visits to key holy sites, including those in Jerusalem.