A major incident has been declared due to "severe flooding" in the basement of the women's and children's unit at Blackpool Victoria Hospital.
Mothers and their newborn babies reportedly had to be moved to different sections of the hospital as chief executive Trish Armstrong-Child said the flooding led to a "loss of power and network access to the entire building".
A back-up generator is said to have failed to start, leaving medical machines unable to function in the maternity ward, including the special care unit.
The cause of the flooding has not been confirmed, but reports suggest the power outage was caused by an overspill of sewage which came into contact with the electrical distribution board for a large section of the hospital.
The overspill is said to have been caused by heavy rainfall on Monday night.
Damian Vanner, a father-to-be, told the Blackpool Gazette he witnessed scenes of "complete chaos" in the unit.
He said: "On entering the ward this morning it looked like a major crisis.
"My partner has had to make a very uncomfortable evacuation from the maternity ward and move to an already overwhelmed day surgery ward.
"After speaking with staff and overhearing rushed working contractors, the conclusion appears to be that an overspill of sewage has come into contact with the electrical distribution board for a large section of the hospital.
"It just looked like complete chaos."
Ms Armstrong-Child said patients have been contacted to rearrange their surgery appointments where necessary. Those who have not been contacted can attend as originally planned.
She added that all patients within the hospital are "safe and continue to receive appropriate care".
The Lancashire Fire and Rescue Service has been working to "reduce localised flooding" but further work is required to clear the area of standing water, the chief executive added.
The hospital has said its "urgent care pathway" is under "a lot of pressure" and has urged people to think before attending the emergency department. They are advised to dial NHS 111 for advice and guidance.