The bomb was found at George V Dock yesterday during planned work at the east London airport, the Metropolitan Police service said.
The airport was shut at 22:00 GMT yesterday and the police said it was working with the Royal Navy to remove the bomb.
The airport will be shut all day and all flights are cancelled, affecting up to 16,000 passengers, a spokeswoman said.
"I urge any passengers due to fly today not to come to the airport and to contact their airline for further information," he said in a statement.
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"I recognise this is causing inconvenience for our passengers, and in particular some of our local residents. The airport is cooperating fully with the Met Police and Royal Navy and working hard to safely remove the device and resolve the situation as quickly as possible," he added.
"The timing of removal is dependant on the tides, however, at this stage we estimate that the removal of the device from location will be completed by tomorrow morning," a statement issued by the Met Police said.
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