Heathrow Airport has warned access to Terminals 2 and 3 is restricted.
Flooding in the central terminal area means anyone travelling to or from either of these terminals must not travel by road.
Passengers are now being strongly urged to use rail and underground services instead.
The flooding is believed to have been caused by a burst main beneath Inner Ring Road West.
It is understood that there is no flooding within either terminal, and flights have not been disrupted.
Those who must travel by car are urged to use Park and Ride, head to Terminals 4 or 5 and take free rail services from there, or change at local rail stations.
"Our teams are working to resolve the situation as quickly as possible, we apologise for any disruption caused," the airport added.
It comes during the school summer holidays, which mark the busiest period of the year for Heathrow and airports widely.
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Rahel Staeheli, 42, and her family have been staying in central London and were due to fly from Heathrow to Vancouver today.
She said her family had initially planned to get an Uber to the airport but "navigated" the Tube stations instead, adding: "Now we're late and hopefully we're going to catch our flight."
Julie Hamilton, 54, has been staying in the Isle of Sheppey and was flying from Heathrow to her home in New Jersey today.
"It's really frustrating," she said.
"I have to be here three hours early and now I'm half an hour late."
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