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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It also comes after the Civil Aviation Authority (CAA) announced it has permitted Heathrow Airport to recoup £320 million spent on its proposal to build a third runway since the start of last year.
Last November, the government supported a £33bn proposal for a third runway, which Heathrow said will be 100% privately financed, through higher airline costs, and no taxpayer money will be used to build the runway or the associated infrastructure.
The CAA's decision means that Heathrow will recover the money through higher airline charges - typically passed on to passengers in air fares, for about 20-25 years.
It added that said its decision will increase the maximum airport charge per passenger by about 15p in 2028, rising to an estimated 30p in subsequent years.
(c) Sky News 2026: Heathrow warns of disruption at two terminals after flooding
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