Hundreds of workers at South Central Trains are to be balloted for industrial action in a row over the use of agency staff.
The Rail Maritime and Transport (RMT) union said 750 of its members, who work as conductors, revenue protection officers and booking office staff, will vote over the next few weeks.
The union said the ballot was being held because of plans to bring in up to 70 agency staff to do revenue protection work normally undertaken by guards and other staff.
South Central runs train services across Sussex.
RMT assistant general secretary Pat Sikorski said: "This is the thin end of the wedge which would lead to the casualisation of the majority of South Central jobs.
"It would mean lower pay, longer hours and worse conditions throughout the company.
"It would also result in the loss of experienced staff with knowledge of railway safety regulations and, at a time of heightened security fears, that is utterly irresponsible."
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