The airport was closed and evacuated earlier this afternoon with at least 27 people receiving treatment
Police investigating a suspected chemical incident at London City Airport believe they have found CS gas spray at the scene.
The airport has now reopened after dozens of people were treated with "breathing difficulties" earlier this afternoon.
The incident saw the airport closed and evacuated while paramedics treated 27 people and the London Fire Brigade (LFB) carried out "protective sweeps" of the airport building.
Two people were taken to hospital.
The airport was declared safe after fire fighters and police wearing protective equipment carried out two complete sweeps of the airport building.
A spokesman said the incident was declared over when, "no elevated readings were found and the building was ventilated, searched and declared safe."
However a subsequent police search of the airport led to the discovery of what is believed to be a CS gas spray.
"Whilst the cause of the incident has not yet been confirmed, officers are investigating whether it was the result of an accidental discharge of the spray," said a police statement.
"Enquiries continue. This is not being treated as a terrorist-related incident. At this early stage officers believe that the spray may have been discarded by a passenger prior to check-in."
A spokesman for the airport said the evacuation took place at about 16:00 BST after a fire alarm was set off.
Around 500 people were told to leave the airport and Transport for London have been warning travellers to expect "long delays."
London City tweeted: "Airport has now reopened. We thank passengers for their patience, access to terminal will be staggered & disruption to flights expected."
London Ambulance's Assistant Director of Operations Paul Gibson, said: "We were called at 4.11pm to reports of an incident at London City Airport.
"We sent a number of resources to the scene including two single responders in cars, four ambulance crews, an incident response officer and our Hazardous Area Response Team.
"We are treating 26 patients at the scene for difficulty [with] breathing; two patients have been taken to hospital."
The LFB said a range of specialist appliances were sent to the scene with scores of passengers reportedly moved on to the tarmac outside the terminal building.
Passengers at London City evacuated onto Tarmac. Police appealing for those affected by chemical inhalation to come forward ⚠ pic.twitter.com/Tgf0okHl1Q— Flight Alerts ⚠ (@FlightAlerts777) October 21, 2016
British Heavyweight boxer David Haye was caught up in the confusion as he attempted to make his way to Scotland.
The cause of the apparent breathing difficulties is still not known and authorities are now investigating.