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.
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.
On 19 July, Heathrow saw 267,000 passengers travel through its terminals, making it the busiest day of the year so far.
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