Emeritus Pope Benedict is serious ill after returning to the Vatican following a visit to Germany to see his dying brother.
The retired pope, 93, is suffering from shingles and has become so weak that he cannot raise his voice, according to his German biographer.
Benedict's 'mind and memory are active but his voice is barely audible at the moment,' author Peter Seewald told newspaper Passauer Neue Presse.
Seewald said Benedict was optimistic when the pair spoke on Saturday, when he said he might pick up writing again if his strength returns.
But his condition has since 'become extremely fragile', the newspaper reported.
Benedict came to his native Bavaria in June to pay his ailing brother Georg Ratzinger a final visit.
Ratzinger, aged 96, died on June 29 after suffering a 'serious illness' which his church did not name.
It was Benedict's first trip outside Italy since 2013, the year he resigned the papacy due to health concerns.
Benedict had previously praised his elder brother for being a 'companion' and 'trustworthy guide' during his life.
The pair had been ordained together on the same day in 1951 after deciding from a young age that they would dedicate their lives to the church.
Georg, who was an organist and choirmaster as well as a priest, confessed that he was 'disappointed' after hearing his brother had been made pope in April 2005 because it would mean they would see less of each-other.
Benedict, who until his appointment had been theological enforcer to John Paul II, held the role until 2013, when he became the first to resign the title in 600 years.
Since his resignation he has lived in a small, former monastery inside the Vatican.
Despite writing several high-profile religious papers since his resignation, he has largely avoided the public eye.
This article has been adapted from its original source.
