Hundreds of rail services will be cancelled on Wednesday as conductors launch a fresh strike in an increasingly bitter row over the role of conductors.
Members of the Rail, Maritime and Transport union at Southern Railway will stage a 24-hour walkout which will cause travel chaos on some of the busiest routes in the country.
The company warned there will be no service on some routes and a limited number of trains on others, describing the disruption as "totally unnecessary".
Southern said it expects to run two thirds of its 2,100 services, but warned there will be a "significant" impact all day.
Several routes will have no service, with only a limited number of trains between 7.30am and 6pm on others.
The two sides blamed each other for the dispute, and for lack of progress since a previous strike last month.
The union published a survey showing that most rail passengers opposed plans to remove guards from trains.
The RMT said the poll of Southern passengers found that three out of four were concerned about the safety of travelling on trains which did not have a guard, and a similar number opposed the removal of guards so that trains were driver-only.
MPs in the union's Parliamentary group have tabled a motion which reads: "At a time of record growth in passenger numbers, passengers want more staff, including a guard on every train, and keeping ticket offices open."
RMT general secretary Mick Cash said: "It is clear from the polling evidence that the public support RMT's safety case for the retention of the guards and an end to the outrageous profiteering on Southern services which sees these vital services used as nothing more than a cash-cow by French state railways to subsidise their operations over the Channel.
"Our members have been backed into a corner by this aggressive and unpopular company and have had no option whatsoever but to fight to defend the safety-critical role of the guard on these rammed-out and unreliable Southern routes.
"This dispute is a battle between the very basic principles of rail safety and the relentless drive by Southern to milk the franchise for every penny they can get."
Southern has been handing out leaflets to passengers warning of the disruption caused by the strike as well as from "unprecedented" levels of sickness.
The company says it is "evolving" the role of conductors on some trains so on-board staff will no longer be responsible for closing doors.
The changes will provide benefits to passengers because there will be more visible staff able to help, say Southern
The company maintains that over 40% of its trains already run with drivers responsible for operating the doors.
The RMT accused Southern of a "disgraceful campaign of misinformation" on a staffing "crisis" that has led to services being disrupted, claims the company denied.
Dyan Crowther, Chief Operating Officer of Southern's owners, Govia Thameslink, said: "Despite repeated efforts over six months, the RMT seem unwilling to talk properly about this. Our door remains open to talks but they seem determined to inflict another day of misery on Southern commuters.
"The only thing that changes is the new conductors will no longer close the doors, a task that passes to the driver with the aid of CCTV. This will cost no-one their jobs, and frees up staff on board trains to better serve passengers.
"We wholeheartedly agree with our passengers who want staff to remain on trains and that's exactly why as many services will have staff on board as they do today.
"Our on-board staff will have a better role which better meets the needs of passengers, securing their valued position on the railway for the long-term."
Southern said the change to the conductor role will result in fewer train cancellations.