Train services to and from London Gatwick will be affected by industrial action planned for this summer.
The disruption starts tomorrow when ASLEF union train driver members working for Southern and Gatwick Express introduce an overtime ban.
The ban will continue until further notice.
Southern Rail said it would only be able to operate 75% of services on Thursday and Friday but would try to get more services running by Monday.
There will be no off-peak Gatwick Express trains between the airport and Brighton, but a limited peak service will operate.
Gatwick Express services will operate a reduced timetable between the airport and London Victoria.
Southern passenger services director Angie Doll said: "This action is going to be very inconvenient to our passengers and communities but by putting in this revised timetable we will be able to run a more reliable and consistent service.
"We urge ASLEF to put an end to their industrial action and allow passengers to get on with their lives."
Thameslink services are not directly affected by the industrial action, although these services are expected to be extremely busy.
Meanwhile, the RMT union has announced a new 24-hour strike between 00:01 and 23:59 on Monday July 10 in the on-going disputes over the safety impact of the extension of driver-only trains and the removal of guards from services.
RMT general secretary Mick Cash said: "RMT members on Southern Rail have been fighting for safety and access for well over a year now.
"The franchise continues to lurch from crisis to crisis as the real impact of hacking back on safety and access hits home."
He said there was ‘absolute chaos’ at Victoria and East Croydon last week in the hot weather, which ‘has reinforced the need for safety critical staff on our trains and platforms’.
"The Government cannot spin their way out of this chaotic situation which is wholly of their making," he said.
"It is now down to Southern/GTR, and the contract holders in the minority Government, to face up to their responsibilities and engage in genuine and serious talks that address our issues."
