Cairo's Torah prison officials said that 84-year-old Mubarak had collapsed several times and had to be placed on ventilators for breathing disorders.
Mubarak's elder son, Gamal has been moved to a ward next to the hospital to allow him to monitor his father in the intensive care unit.
Speaking on condition of anonymity, the officials said Mubarak has lost weight and is having difficulty walking, but doctors managed to convince him to take his medicine, which he was initially refusing.
Doctors are submitting a medical report to Interior Minister Prisons Department Chief Mohamed Naguib, who would hand them over to the public prosecution.
The former president has not been allowed to keep his personal guards in the jail hospital.
Mubarak was sentenced on Saturday for failing to stop killings of protesters during last year's mass uprising that toppled him. His conviction saw him transferred to prison.
Earlier, he had been in hospitals, first in his favorite Red Sea resort of
Sharm el-Sheik and later at a suite in a military hospital on the outskirts of Cairo.
Mubarak's sons Alaa and Gamal were acquitted in corruption charges, but are still held over a separate trial on charges of insider trading.