The Gherkin was evacuated this morning, after emergency services were called to reports of smoke in the basement of the City building.
London Fire Brigade 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."
The smell of smoke was caused by fuse in a transformer which had burned out.
The Brigade was called at 08:38 and the incident was over at 09:42. Crews from Whitechapel, Dowgate, Shadwell and Dockhead fire stations were at the scene.
City of London Police said it was "not a police incident".
The iconic skyscraper, which is officially known as 30 St Mary Axe, contains 33 floors of offices.
Witnesses posted photos on Twitter of fire engines outside the distinctively shaped building.
No sign of any incident but the Gherkin has been evacuated. I guess security at notable buildings very cautious today. pic.twitter.com/cjaI9JKZm0— tristan (@tristanpc) March 23, 2017
The City of London is on high alert after yesterday's terrorist attack at Westminster, in which four people were killed. Seven people have been arrested in connection with the incident.
After the violent events at Parliament, the City of London Police announced there would be extra officers on the ground in the Square Mile.