Former Pope Benedict XVI, the first pontiff in 600 years to resign instead of ruling for life, is seriously ill, a German newspaper reported on Monday.

The newspaper, Passauer Neue Presse, cited Benedict's biographer, German author Peter Seewald, who met the 93-year-old emeritus pope at the Vatican on Saturday.

A Vatican spokesperson had no comment on the report and the former pope's personal secretary, Archbishop Georg Ganswein, did not immediately return a call asking for comment.

Seewald said Benedict, who has been in frail health for some time, was now suffering from shingles, a viral infection that causes painful rashes and is common among older people.

Seewald said Benedict's voice is barely audible — echoing something other visitors have said for months — and that the former pope told him he might pick up writing again if he regains his strength.

In June, Benedict, who lives in a former convent in the Vatican gardens, left Italy for the first time since his resignation in 2013 for an emotional farewell visit to his older brother Georg in their native Bavaria.

Georg Ratzinger died on July 1 aged 96. The two brothers were ordained priests on the same day in 1951.

Benedict, who was elected in 2005 to succeed the widely popular Pope John Paul II after a reign of nearly 27 years, shocked the world and even his closest aides on February 11, 2013, when he announced in Latin he was stepping down.

He told a gathering of cardinals that he was too old and frail to lead an institution with more than 1.3 billion members.

At the time of Benedict's resignation, the Vatican was mired in a raft of financial woes, sexual abuse scandals, and infighting among bureaucrats that led to leaks of important documents.

Benedict said his resignation was exclusively due to health reasons.

Hardline conservatives in the Church, who have been alarmed by Pope Francis' progressive moves, have looked to Benedict as their standard bearer.

This forced the former pope on several occasions to remind them that there is only one pope — Francis.