Watford General Hospital has declared a critical incident after a technical incident with oxygen equipment.
Hospital bosses say emergency patients should not come to its accident and emergency department.
They have advised people to go to different A&E units as the situation is currently being assessed by technicians.
Patients receiving treatment there are not at risk and their situation is not changing.
A spokesperson from the hospital said: “As a result of a technical issue with our individual hospital’s oxygen equipment, which does not pose any risk to our patients, West Herts Hospitals NHS Trust declared a critical incident on Saturday, April 4 and has asked that people do not attend Watford General Hospital.
“A small number of patients are being transferred to other hospitals in the area, with each patient being fully assessed in line with existing safety guidelines before they are moved.”
The hospital is currently closed to all patients except women in labour and a small number of patients are being transferred to other hospitals.