The entire staff of Croydon Council spilled out onto the streets as part of a mass evacuation from its Taberner House headquarters yesterday.
It was initially thought that a fire had started in an elevator. But it is now known that an electrical panel had overheated and started smoking.
Pupils from St Mary's Catholic High School, Woburn Road, who had been on work experience placements in the building were also evacuated.
An eyewitness said: "Everyone was marched out onto the street and fire engines turned up soon afterwards, even the poor work-experience kids had to line up in the cold."
St Mary's headteacher, Ejiro Ughwujabo, said: "I spoke to all of the four-students I have on placements at the council and they are all safe and sound. None of them were hurt and they will resume their work experience."
A council spokesman said: "At approximately 2.30pm yesterday Taberner House was evacuated and the fire brigade called to investigate reports of a smell of smoke from the motor and control panel room.
"The fire brigade found no fire but reported the cause of the smell to be that of the control panel overheating."
Little more than 20-minutes after staff were allowed back in Taberner House another fire-alarm went off at the town hall in Katharine Street after a water leak dripped into a power supply panel and activated the smoke alarms.
Staff, along with members of public, were allowed back into the building at around 4.05pm after emergency services declared the building safe.
A spokesman from Croydon fire station said: "We were called to reports of a fire in Taberner House at approximately 2.34pm. Four fire engines attended but no fire was found. We believe a control panel overheated causing it to smoke."
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