All flights in and out of travel hub were diverted for six hours when activists chained themselves together on a runway.
London City Airport has been declared safe after hundreds of passengers and staff were evacuated following a reported “chemical incident” that left 26 travellers feeling unwell and two taken to hospital.
The London Fire Brigade said three fire engines had been sent to the scene following “reports of a chemical incident”, but did not say what the cause might be.
But the airport was declared safe after emergency services entered.
“Two complete sweeps of the airport building were carried out jointly by firefighters and police officers, both wearing protective equipment,” the fire brigade said. “No elevated readings were found, and the building was ventilated, searched and declared safe.”
The airport was declared “safe” around 19:00 local time (18:00 GMT) but the police reported possible traces of CS gas spray.
“A search of the airport led to the discovery of what is believed to be a CS gas spray,” a Metropolitan Police statement read.
“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’s official Twitter account said it had been evacuated “due to fire alarm”.
“We are treating 26 patients at the scene for breathing difficulty,” the London Ambulance Service said earlier.
A “hazardous area response team” was also sent to the airport in east London, which caters mainly for business travellers and short-haul flights.
“Around 500 members of the public and staff have been evacuated,” the fire brigade added.
Several incoming flights from destinations such as Frankfurt, Amsterdam and Paris were diverted to other airports. The airport announced it had reopened about three hours after the incident.
London City Airport is primarily aimed at business travellers and is the 13th busiest airport in Britain, with a total of 4.3 million passengers last year.