Two people have been taken to hospital and dozens treated at London City Airport after reports of a "chemical incident".
Police have found a tear gas canister and are investigating whether this was accidentally let off, the Daily Mail repo.
Hundreds of people were evacuated from the airport this afternoon and the capital remained on high alert following a terror alert at a nearby tube station just 24 hours ago.
The airport has now re-opened, but disruption is expected long into the evening.
A spokesperson for London City Airport said: "We apologise to passengers for the inconvenience caused today when an alarm was activated, triggering a full evacuation of the airport terminal.
"Passengers were evacuated safely and we thank them for their patience. Following the evacuation, some individuals reported feeling unwell and were treated at the scene by London Ambulance Service.
"Emergency services responded to the evacuation, citing a possible chemical incident with firefighters and police officers jointly conducted sweeps of the airport building.
"The 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 yet to be confirmed, officers are investigating whether it was the result of an accidental discharge of the spray.
"The airport was declared safe and reopened at approximately 1900. Passengers are advised to contact their airline for the latest information regarding their flights."
Incoming international flights were diverted away from the airport and passengers were left stranded on planes that had already landed.
It is not yet known how long it will take for normal service to be resumed.
However, London Fire Brigade have released a statement declaring the area safe.
A spokesman said: "Two complete sweeps of the Airport building were carried out jointly by firefighters and police officers both wearing protective equipment.
"No elevated readings were found and the building was ventilated, searched and declared safe."
Pictures from the scene show crowds gathering on the tarmac and the runway.
All flights were suspended and several incoming planes from destinations such as Frankfurt, Amsterdam, Belfast City and Paris were diverted to other airports.
Paul Gibson, Assistant Director of Operations for the London Ambulance Service, said: 'We have now treated a total of 27 patients at the incident at London City Airport this evening.
"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 (medics specially trained to treat people in hazardous situations).
"All patients were treated for minor breathing difficulties and two were taken to hospital.
"The incident is now stood down."
A Metropolitan Police spokeswoman said: "Police were called shortly after 4pm to London City Airport to reports of a number of passengers feeling unwell.
"The cause of the incident is not yet known. The airport has been evacuated as a precaution."
Boxer David Haye, who was at the airport to catch a flight to Scotland, tweeted that the airport was evacuated when "everyone started coughing uncontrollably!"
A witness who was evacuated from the airport told LBC radio that blankets have been given to children and that food and water is being given out.
And a taxi driver who was waiting outside the terminal was asked if the back of his cab could be used to treat someone who was taken ill with "itchy eyes".
TFL said in a tweet: "London City Airport is CLOSED due to a police incident. Connaught Rd, Connaught Bdg Rbt are SHUT. Pls use other routes. Expect long delays."