Hundreds of City workers were evacuated from the Gherkin amid reports of a fire in the basement of the skyscraper.
London Fire Brigade were called to St Mary’s Axe, in the capital’s financial district, on Thursday.
City workers were herded out of the building by fire crews just after 8.30am.
Dramatic images posted on Twitter showed crowds gather outside the building.
A spokesman for London Fire Brigade confirmed the tower had been evacuated.
He said crews were investigating the smell of smoke from a generator room.
The Brigade later confirmed the smoke was caused by a burnt out fuse.
Staff in nearby offices were told that fire crews had been called to reports of smoke in the building's basement and that workers had been evacuated as "protocol".
Four fire crews, including 21 fire fighters were sent to the scene and workers were allowed back into their officers around 9.45am.
A police cordon was reportedly put in place to control crowds.
Concerned bystanders took to Twitter and described "fire engines and police everywhere".
One user wrote: "Fire engines, police and the streets are full of people. What's happened at the Gherkin?"
Jeremy Brier added: "Apparently the Gherkin has just been evacuated with police cordon."
Joshua Hurrell said: “Walked past the Gherkin being evacuated, three fire engines,hope nothing serious is going on."
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