Two people were hospitalised and dozens treated for breathing difficulties after a canister of CS gas was accidentally discharged at London City airport.
The alert triggered a full evacuation of the Docklands airport on Friday afternoon.
The fire alarm went off at about 4pm, causing the airport to grind to a halt as all flights in and out were stopped and 500 people were made to wait outside.
Incoming aircraft from destinations including Frankfurt, Amsterdam, Belfast City and Paris were diverted to other airports.
Police, the London Fire Brigade and London Ambulance Service were called.
The airport was declared safe and reopened at about 7pm, although the closure caused chaos and delays for passengers.
Paramedics treated 25 people at the scene for breathing difficulties and two members of ground staff were taken to hospital as a precaution, British Airways said.
The airport and Metropolitan police confirmed that a search of the building had found a CS gas spray. The substance is used by police to control violent criminals and causes streaming eyes, a burning sensation and blurred vision. It is classed as an illegal weapon.
Early indications suggested that the substance may have been left by a passenger before check-in, The Times reported.
A police spokesman said: “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.”
London City airport apologised to passengers for any inconvenience.