Four fire engines and around 25 firefighters were called to reports of smoke at an office block on Smith Square, Westminster.
Firefighters investigated and discovered a small fire in a service duct on the 7th floor.
Approximately 300 people left the building and adjacent buildings before the arrival of the Brigade.
The Brigade was called at 1238 and the incident was over for firefighters at 1357. Fire crews from Soho and Lambeth fire stations attended the scene.
The cause of the smoke is under investigation by the Brigade’s fire investigation team.