Rail passengers on some of the busiest routes in the country face travel chaos because of a conductors' strike.
Southern Railways warned it could not guarantee that travellers will get to their destination because of the "significant" impact on services.
Members of the Rail, Maritime and Transport (RMT) union will walk out for 24 hours from 11am in a dispute over the role of conductors and driver-only trains.
Southern said no services will run on many routes and only a limited number on others.
Queuing systems will be in place at many stations that are open, while picket lines will be mounted by strikers.
Two further 24-hour strikes are planned next month on Southern, which runs services across the south of England, including commuter routes into London Victoria.
Southern, owned by Govia Thameslink Railway (GTR), plans changes to the role of conductors which will see responsibility for operating the doors switched to the driver.
Mick Cash, RMT general secretary, said: "This dispute is about safety and the safety-critical role of the guards on Southern trains.
"The company, with an eye on ever-fatter profits, is prepared to axe the guards on some of the most overcrowded and potentially-dangerous services in Britain so that they can squeeze every last penny out of their passengers regardless of the consequences.
"That is a lethal gamble with safety in the name of profit and that is why we have been forced to take strike action.
"Southern see their passengers as nothing more than self-loading freight, there to be fleeced at every turn and every opportunity.
"The consequences of removing guards, the human eyes and ears on that dangerous interface between crowded platforms and crowded trains, is blindingly obvious to anyone but the rip-off merchants running this deeply unpopular rail franchise.
"The company have now resorted to trying to bully and intimidate staff taking a stand to defend the safety-critical role of the guard. That is disgusting, but nothing surprises us with this mob."
Transport Secretary Patrick McLoughlin said: "I condemn any industrial action that will disrupt the travelling public. I expect to see operators and the unions working together to resolve these issues.
"Rail passengers will not thank the unions for inflicting this unnecessary disruption. It is clear that the changes GTR are proposing will modernise services and provide better journeys for passengers."