All flights coming in and out of the airport, in east London, will be stopped after the device was found nearby in the River Thames on Sunday.
The closure led to the cancellation of more than 100 departures and was affecting up to 16,000 passengers, according to a spokeswoman.
The device was discovered at the George V Dock at about 5am yesterday at the airport.
Newham Council said the bomb was a German 500kg fused device and warned work to deal with it would last until Tuesday.
The Metropolitan Police said the airport was shut at 10pm, as officers worked with the Royal Navy to remove the bomb.
