Police called for the suspension of operations at the airport, located near London's Docklands area, when workers at a nearby building site discovered the bomb, a statement on the airport's Web site said.
The area has been cordoned off and local train services have suspended too, according to a spokeswoman for the Metropolitan Police who spoke on condition of anonymity in line with force policy.
Airport officials asked passengers to check with airlines for further travel information. They provided no information about when the airport was likely to be reopened, the AP reports.
The area where the bomb was discovered is in a part of London that was heavily bombed during World War II.
London City airport served two million passengers last year.
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