Georg Ratzinger, told CNN in Regensburg, Germany, that Benedict had come to his decision without seeking his brother's advice.
CNN's Sarah Brown and Richard Allen Greene in London and Isa Soares in Regensburg, Germany, contributed to this report.
So it's also just like the Pope's Regensburg speech back in 2006, remember?
Regensburg once was a Roman garrison town and is a celebrated tourist site.
Joseph Fessio, a theology student of Ratzinger's at the University of Regensburg from 1972-74, and now the English-language publisher of his books.
It is not clear if there is any link between the Leipzig scandal and manipulations uncovered previously in Munich, Regensburg and Goettingen.
Speaking to the BBC from his home in Regensburg in Germany, Georg Ratzinger said his brother's resignation was part of a "natural process".
In September 2006, Pope Benedict XVI provoked outrage in the Muslim world with a speech given at the University of Regensburg in Germany.
Georg Ratzinger, speaking in Regensburg, Germany, said he expected to see a pope chosen from outside Europe one day -- but not just yet.
REGENSBURG, GERMANY Washington has sharply reduced the number of American combat forces in Europe since the end of the Cold War, but a large U.S. military footprint remains.
They moved into their 500-room castle in Regensburg in 1812.
Benedict spent the bulk of his early career in the classroom, as a student and then professor of dogma and fundamental theology at universities in Bonn, Muenster, Tuebingen and Regensburg, Germany.
Certainly his controversial wording about Islam in his speech at Regensburg in 2006 was an example of a scholar pope unable to reach a mass audience, especially when compared to the media maestro John Paul II.
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.
BBC: Pope Benedict 'will not interfere in successor's affairs'
In the middle of a dense, intellectual, argument about faith and reason, laid out in a speech at Regensburg University in his native Germany, he made reference to a dialogue between a Byzantine ruler, Manuel II Paleologos, and a Persian scholar.
"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 told the BBC from his home in Regensburg, Germany.
Members of other faiths will, perhaps, always think of him in contradictory terms - immensely charming and gentle in his personal dealings with Muslim and Jewish leaders and yet capable of huge PR gaffes, such as the now infamous Regensburg speech of 2006 in which he was accused of describing the prophet Muhammed in less than flattering terms.
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