The emergency department at the Conquest Hospital in St Leonards has been temporarily closed.
Managers this morning urged patients needing urgent treatment to travel to other hospitals instead or call NHS Direct on 0845 4647 for information.
The move was made last night due to a "structural issue" with the incinerator on the industrial part of the hospital site.
Accident and Emergency and the Emergency Assistance Unit departments have been closed and ambulance patients will be taken to Eastbourne District General Hospital and other hospitals in the area.
An East Sussex Hospitals NHS Trust spokesman said: “The situation is under control and no patients are at risk.”
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