Heathrow Airport's control tower was evacuated after a suspect car sparked a security alert.
No planes were able to take off or land in the world's busiest international airport for 20 minutes before British police gave the all clear.
Access to Terminals One, Two and Three was also blocked for more than 30 minutes after Heathrow tunnel was closed in the alert.
A spokeswoman for the airport says many flights are now running about 45 minutes late, although services are expected to be back to normal by the end of the night.
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A Scotland Yard spokeswoman says the alert was sparked after the car was found on airport's perimeter road.
"It has now been checked and has been removed as a security measure. It did not pose any danger," she added.
PA