Francis was admitted to Rome’s Gemelli hospital on February 14 after experiencing breathing difficulties.
Pope Francis’s health has deteriorated over the past 24 four hours, the Vatican said, adding that the pontiff had suffered a “prolonged asthma-like respiratory crisis” and needed blood transfusions.
“This morning Pope Francis presented a prolonged asthmatic respiratory crisis, which also required the application of high-flow oxygen,” the Vatican said on Saturday.
“The Holy Father remains alert and has spent the day in a chair, though he is more unwell than yesterday,” it added.
Francis was admitted to Rome’s Gemelli Hospital on February 14 after experiencing difficulty breathing for several days. He has since been diagnosed with double pneumonia.
The pontiff’s doctors had told a news conference on Friday there was no imminent risk to his life but that he was “not out of danger”.
But on Saturday, the Vatican said that “the Holy Father’s condition remains critical” and added that doctors had to administer a “high flow” of oxygen because of his breathing crisis. It also said that the blood transfusions were necessary because tests showed he had a low platelet count, associated with anaemia.
Double pneumonia is a serious infection that can inflame and scar both lungs, making it difficult to breathe.
Francis has continued to work during his hospital stay, including making bishop appointments.
Earlier on Saturday however, the Vatican announced that the pope would not appear in public on Sunday to lead prayer with pilgrims, the second consecutive week he will have missed the event.
There is no provision in canon law for what to do if a pope becomes incapacitated.
Pope Benedict XVI resigned in February 2013 citing declining health, but Francis has shown no signs of stepping down.