Benedict, 95, became the first leader of the Catholic Church to stand down in 600 years in 2013, citing advanced age.
At the end of the Pope’s final audience of the year, he asked people to “pray a special prayer for Pope Emeritus Benedict”.
The Vatican then said the ex-Pope’s health had worsened in recent hours.
“The situation at the moment remains under control, constantly followed by doctors,” said spokesman Matteo Bruni.
Pope Francis was addressing a general audience in the Vatican’s Paul VI hall when he looked up from a piece of paper and spoke about Benedict’s declining health.
He then went to visit him at the Mater Ecclesiae monastery, where Benedict has lived since he stepped down.
Earlier this month Francis revealed he frequently visited his predecessor.
Speaking of Benedict as a “saint” and a man of high spiritual life, he said the former pope was lucid and had a good sense of humour.
“He speaks softly but follows your conversation,” he told Spanish newspaper ABC.
Benedict XVI was 85 when in February 2013 he surprised Catholics around the world with his decision to step down, less than eight years after he had been elected Pope as Cardinal Joseph Ratzinger.
Not since Gregory XII stepped down in 1415 had a pope resigned.Tags: