Bob Crow, general secretary of the RMT union, is demanding assurances that jobs, pensions and benefits will not be adversely affected when the state-run South Eastern Trains (SET) is re-privatised early next year.
Mr Crow expressed concern that new private-sector management aimed to cut costs by shedding employees and undermining the terms of those who were left. He had failed to win the guarantees he sought.
Representatives of the RMT are confident that the 1,350 members concerned will back industrial action, causing major operational problems. The ballot is due to begin in about a week and the first stoppage could be scheduled for October.
Mr Crow has led opposition to the re-privatisation of the franchise, saying the service, which covers Kent and parts of Sussex, has improved "beyond recognition" since the French operator Connex was stripped of the franchise.
The RMT leader said there was overwhelming support for the franchise to remain in public hands. "The last time the franchise was handed to the private sector, staffing levels were pared to the bone, our conditions came under attack, staff morale hit rock bottom, while services deteriorated alarmingly and public confidence evaporated.''
Mr Crow said in the run-up to the re-privatisation of the operation the company was already trying to cut the number of station staff. "Our members' fears are founded on a bellyful of experience of privateers seeking to maximise profits by cutting staff, undermining decent pension schemes and squeezing pay and working conditions while pocketing massive subsidies."
A spokeswoman for the Department for Transport said the issue was a matter for management at SET.
An official at SET said the ballot had been called in advance of a meeting due to take place between management and union representatives this monthto discuss Mr Crow's demands.
Management intends to send out a letter to all employees today putting the company's arguments.