More than 250 staff at the parent company of Southern Railway are voting on whether to take strike action in a dispute over ticket office closures and job losses, rail union TSSA said.
The result of the ballot of ticket office and platform workers at Govia Thameslink Railway (GTR) will be announced on September 2.
The union said GTR plans to close or partially close more than 80 ticket offices and axe over 40 staff.
TSSA general secretary Manuel Cortes said: "We cannot stand by and allow GTR to reduce Southern Railway to a second-class operation which is not fit for passengers and staff.
"Passengers have suffered enough over the past few months and it is now time for (Transport Secretary) Chris Grayling to act.
"GTR should be stripped off this franchise and it should be handed over to the publicly-run Transport for London as (London Mayor) Sadiq Khan has suggested.
"Then we will have a publicly-run franchise which will be operating in the public interest, not in the interest of private shareholders."