Emeritus Pope Benedict is reported to be seriously ill after a visit to Germany to see his dying brother.
Benedict, 93, paid a final visit to Georg in Bavaria in June shortly before his death that month.
Biographer Peter Seewald told German newspaper, the Passauer Neue Presse that the retired Pope was suffering from shingles and was unable to raise his voice.
While Benedict was said to have an "active" mind and memory, his voice has become "barely audible at the moment" and his condition was described as "fragile".
Seewald said he had spoken to Benedict on Saturday and that the retired Pope had expressed a desire to start writing again if his health improves.
Benedict stepped down as Pope in 2013, the first Pope to do since Gregory XII in 1415.
At the time, he cited declining health due to old age. Although he relinquished leadership of the Catholic Church, he was given the honorary title of Pope Emeritus and continues to reside in Vatican City.