Gatwick warned holidaymakers to allow extra time for their journeys due to a five day strike affecting Southern Rail services to the airport.
At least 40% of Southern services in the southeast have been cancelled until Friday after talks with the Rail, Maritime and Transport union collapsed last week.
Gatwick Express and Thameslink services to the airport are to operate with amended timetables.
But travellers were urged to allow extra time for their journeys to the airport.
Southern, which is owned by Govia Thameslink Railway, has warned passengers of overcrowded services, with no trains on some routes in Sussex and shortened operating hours on others.
Services will be "significantly affected" with an amended timetable put in place and passengers were advised to travel outside peak periods.
First-class carriages will be made available to standard ticket holders to squeeze in more passengers but many routes, including the London to Brighton line, are expected to be “very busy”.
Conductors are going on strike as part of a dispute over plans to turn them into “on-board supervisors”, leaving drivers to open and close train doors at stations.
Southern operates through 156 stations and has 620,000 passenger journeys per day, according to its website.
Rail minister Paul Maynard said: “This strike action from the RMT will cause yet more disruption and daily misery for passengers.
“It is deeply disappointing that the union bosses continue to overlook the impact they are having on the travelling public, and I strongly condemn this proposed action.”
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