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 made the short trip from the hall to the Vatican Gardens to see Benedict at the Mater Ecclesiae monastery, where he 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.
The former Pope has struggled with speech for some time and two years ago a Maltese cardinal said Benedict had told new cardinals that "the Lord has taken away my speech to let me appreciate silence". "He speaks softly but follows your conversation," Pope Francis told Spanish newspaper ABC.
This is believed to be the most recent photograph of Benedict, here meeting Pope Francis at the monastery in August
Cardinals around the world joined Pope Francis in praying for his predecessor. "In these difficult and serious moments, let us unite in fervent prayer for our dear Pope Emeritus," wrote Guinean Cardinal Robert Sarah, the most senior African prelate at the Vatican until last year.
When the Pope took new cardinals to meet him at the Mater Ecclesiae monastery in August, Benedict looked frail but he shook them all by the hand and engaged with them.
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.
Benedict had presided over his predecessor John Paul's funeral Mass. He was the eighth German to become Pope and was known for his conservative, traditionalist views, campaigning against the social activism of liberation theology.
But his papacy was marked by a scandal involving child sexual abuse by priests. Two reports in 2009 detailed the extent of paedophilia and cover-ups within the Irish Church - and it later emerged that almost 400 priests had been defrocked by Benedict in 2011 and 2012.
However, earlier this year the former Pope accepted that errors had been made in handling sexual abuse cases while he was archbishop of Munich from 1977 to 1982.
A German inquiry commissioned by the Church found that he had failed to act in four cases. Benedict denied wrongdoing but asked forgiveness for any "grievous fault".
Cardinal Reinhard Marx, the current archbishop of Munich, said he had seen Benedict in September and called on churchgoers to include him in their prayers.
Although Benedict was the first Pope for 600 years to stand down, Pope Francis made clear in his Spanish interview this month that he too had signed a letter of resignation in 2013 "in case of health impediment or whatever", handing it to Cardinal Bertone, the Vatican secretary of state at the time.