City Airport has been closed after a World War Two bomb was found nearby in the River Thames.
The airport will be shut all day and all flights cancelled, affecting up to 16,000 passengers, a spokeswoman said.
The bomb was discovered at about 05:00 GMT on Sunday at George V Dock during pre-planned work at the airport in east London, the Met Police said.
The airport was shut at 22:00 and the Met said it was working with the Royal Navy to remove the device.
Passengers have been told not to travel to the airport as the terminal is closed and to contact their airline.
Robert Sinclair, CEO of the airport, apologised and said: “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.”
Airlines using London City Airport include British Airways, Flybe, CityJet, KLM and Lufthansa, with flights to domestic and European city destinations.
Police said a 214-metre exclusion zone had been set up and properties inside were evacuated, with Newham Council providing temporary emergency accommodation to residents.
“While we endeavour to progress the operation as quickly as possible and minimise disruption, it is important that all of the necessary steps and precautions are taken to ensure it is dealt with safely,” the Met said.
“We would like to thank everyone affected for their patience and cooperation.”
Police said a number of road cordons were in place and advised motorists to avoid the area.
Docklands Light Railway services between the airport and Woolwich Arsenal have also been suspended.
Passengers have used social media to express confusion and frustration following the closure.
Twitter user Richard Macey talked about his Dublin flight being cancelled.
Flight to Dublin from @LondonCityAir cancelled due to a WWII bomb, looks like I’ll be working in London today then.
End of Twitter post by @RICHARDMACEY007
Another user said they were attempting to book a new flight to Aberdeen.
End of Twitter post by @bclsays
Last year, more than 4.5 million passengers used London City Airport.
A £400m expansion was given the go-ahead by ministers in July 2016, which includes extending the terminal.
Two million more passengers per year will be able to use the airport from 2025, with 30,000 additional flights annually, the airport said.