Train drivers employed by Southern Railway and Gatwick Express are to be balloted for strikes, the union Aslef has said.
The move follows talks over the imposition of new rosters and a “breakdown in industrial relations”.
Parent rail firm Govia Thameslink (GTR) is already in dispute with the RMT union over the role of guards on the Southern network.
They said it was a cynical and desperate attempt to heap misery on passengers.
The rail operator is currently running a reduced timetable to deal with ongoing disruption.The Aslef dispute is over the new timetable.
The RMT is due to strike for five days from 8 August.
That dispute is over new trains where the driver operates the doors using CCTV instead of guards, but the RMT has safety concerns and fears job losses.
A third union, the TSSA, is balloting more than 250 station staff in a row over ticket office closures.
Commuters have staged protests in London and the South East over continued disruption on the Southern rail network.