Rail passengers will face some of the worst disruption for years from Tuesday when a wave of strikes and other forms of industrial action are launched in bitter disputes over staffing.
Members of the Rail, Maritime and Transport union on Southern Railway will strike for three days in protest against changes to the role of conductors, while the drivers' union Aslef will start an overtime ban ahead of strikes later in the month and in the new year in a row over driver-only trains.
Southern is taking legal action in the High Court on Wednesday to try to stop the strikes, but if the action goes ahead all services will be halted.
There was some relief for commuters hen a threatened strike by Tube drivers in London from Tuesday evening was called off after progress during peace talks.
Southern warned passengers to expect "severe and significant" disruption to Southern and Gatwick Express services every day from Tuesday.
On RMT strike days, only around 50% of the full timetable will operate. If the Aslef strikes go ahead, no Southern services will operate, with only Thameslink services to Three Bridges and Brighton and a limited service on Gatwick Express.
Southern's West London Line services, London Bridge to Beckenham Junction services, and Brighton to Seaford services are all cancelled from Tuesday until further notice.
Buses will operate between Lewes and Seaford. Other metro services will also be reduced.
Southern director Alex Foulds said: "Regrettably, because of this wholly unnecessary and unjustified industrial action, there will be severe and significant disruption on our network from Tuesday and customers are advised that stations will be incredibly busy.
"If passengers can make alternative travel arrangements they should, and if they don't have to travel they shouldn't. If the drivers' strikes go ahead, there will be no services on Southern and customers should not attempt to travel.
"We're doing everything we can to stop the drivers' strike and that's why we are seeking an injunction in the High Court.
"This industrial action is a clearly co-ordinated and cynical manoeuvre by the unions to bring yet further travel misery to passengers, as well as having a detrimental impact on the regional economy when it least needs it.
"If the unions are listening to passengers then they will call off all industrial action now and give hardworking commuters and their families their lives back."
Thameslink is not directly affected by the strike action, although its services are expected to be extremely busy.
Other operators' services are not affected, but are likely to be busier.
TSSA staff working on the Tube will refuse to work overtime from Thursday in a separate dispute over jobs and ticket office closures.