The A&E was shut down and ambulances diverted after reports of a fire at Croydon University Hospital earlier this afternoon.
Firefighters wearing breathing apparatus searched the accident and emergency department using thermal imaging cameras after someone pressed the emergency alarm button.
There was a strong smell of smoke but after looking throughout the department, the lift motor room and the plant room, the crews concluded there was no fire.
The officer in charge of the incident, watch manager Rob Scrivener from Norbury fire station, said: “There was a smell of burning from a heater and it turned out it had overheated.
“The hospital instigated its major incident procedure which diverts ambulances to other hospitals until I could confirm there was no fire.”
The crews from Norbury, Croydon, Tooting and Forest Hill were called at about 12:15pm and were there for 30 minutes.
The A&E is now back to normal.
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