The airport said passengers were told to leave the terminal Friday afternoon because a fire alarm was sounding. Several incoming flights from destinations such as Frankfurt, Amsterdam, Belfast and Paris were diverted to other airports.
The Metropolitan Police force says officers were called to the airport just after 4 PM (1930 IST), "following reports of passengers being taken unwell."
Police, the ambulance service and the London Fire Brigade are all at the airport investigating.
City Airport, located in east London's docklands, is the smallest of the British capital's international airports.
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