The large building at 30 St Mary Axe – nicknamed the Gherkin – in the City of London was evacuted just after 8.30am today.
Fire engines were seen parked outside as hundreds of workers streamed out of the building.
An alarm set off the evacuation just as people were arriving at work.
The Gherkin was evacuated just before 9am
The gherkin in London has been evacuated…is this another attack on London? Scary times
London Fire Brigade rushed to the scene as workers were being evacuated.
An LFB spokesman, said: "We were called at 8.36am.
"We are investigating the smell of smoke from a basement room at the Gherkin.
"Crews are there now.
"We sent four fire engines from four local fire stations."
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Workers took to Twitter as they were told to leave the iconic building, with some fearing a terror incident after yesterday's attack on Westminster in which four people were killed.
One Twitter user said the Gherkin had been on lockdown yesterday following the attack.
The Gherkin was evacuated just after 8.30am
Lizzi D, said: "Fire engines, police and the streets are full of people. What's happened at the gherkin?"
Jay, said: "The gherkin in London has been evacuated…is this another attack on London? Scary times."
Nathan Budd, said: "Not ideal walking to work and seeing the gherkin evacuated with police and fire engines everywhere!"
Gemma McCarthy, said: "The Gherkin's just been evacuated – stay safe my fellow London workers – hopefully it's unrelated to everything that's going on."
Nearly two hours after the initial alert, LFB said crews would be leaving the Gherkin "shortly".
A spokesman, said: "Four fire engines and 21 firefighters and officers were called to investigate a smell of smoke coming from a generator room in the basement of the Gherkin at St Mary Axe this morning.
"A fuse in a transformer had burned out.
"The Brigade was called at 0838 and the incident was over at 0942. Crews from Whitechapel, Dowgate, Shadwell and Dockhead fire stations were at the scene."
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