Approximately 400 people were evacuated from an office block after a chemical incident in Churchill Place, Canary Wharf last night.
The incident is believed to have been caused by a mixture of cleaning products which caused a chemical reaction and produced chlorine gas.
Fire crews carried out a detailed sweep of the building. After finding no elevated readings they declared the building safe.
Four people were treated at the scene for breathing difficulties by London Ambulance Service crews.
The Brigade was called at 1921 and the incident was over for firefighters at 0125. Fire crews from Millwall and Poplar fire stations attended the scene along with fire rescue units from Lewisham and Euston fire stations.