Southern Railway train drivers have started a ban on overtime which will lead to the cancellation of hundreds of services, causing fresh misery for the region's 300,000 passengers.
The action by members of the Aslef union, in a long-running dispute about driver-only operation, will affect a quarter of Southern's trains today and tomorrow.
Southern, which runs more than 2,200 services a day, said it was working to increase the number of trains next week.
The action started ahead of a court bid by the Association of British Commuters (ABC) for a judicial review of the Government's handling of the Southern franchise.
The passengers' group will take its case to the High Court, arguing that a judicial review should be held, following a year of disruption due to industrial action, staff shortages and other problems.
Southern passengers face more chaos on Monday July 10 because of a 24-hour strike by members of the Rail, Maritime and Transport union.