The country's biggest rail franchise, which includes Southern and Gatwick Express, is facing the threat of fresh strikes in a new dispute over jobs and pay.
The Rail, Maritime and Transport union announced it is balloting 1,000 of its members at Govia Thameslink Railway who work on stations, in a row over the impact of plans to close ticket offices.
The union is embroiled in a separate, long-running dispute with Southern over moves to change the role of conductors which has led to strikes.
The RMT said it was totally opposed to ticket office closures and cuts in hours, warning it would have a "devastating" impact on staff and the safety and services offered to the travelling public.
The union has rejected the latest proposals from the company, claiming they cut pay and would result in a massive increase in lone working.
A union spokesman said: "The impact of the proposals cannot be underestimated. GTR wants to close ticket offices, or cut them to morning peak only, at 83 stations from as far afield as King's Lynn and Bognor Regis.
"As a means of doing this they plan to introduce a new multi-functional role of station host which RMT estimates will result in a cut of at least 130 jobs."
The company maintains its plans would lead to more staff on station concourses to help passengers and sell tickets.
Southern services have suffered delays and cancellations for several weeks because of the conductors' dispute and staff shortages blamed on high levels of sickness.
GTR's passenger service director, Keith Jipps, said: "The RMT's threat of further industrial action is entirely unwarranted and clearly another bid by the union to disrupt passengers and GTR across as many parts of our franchise as possible.
"We have listened to passengers and modified our proposals, addressing the concerns of both London TravelWatch and Transport Focus.
"Our new station hosts will be paid more, be able to work in safety and provide passengers with better customer service, but the RMT is not concerned with improving the experience for passengers and are dismissing significant improvements to the terms and conditions for staff."