Southern Rail drivers are to be balloted for industrial action after talks failed to resolve a dispute, according to the Aslef union.
A three day-strike is currently being held by the RMT union, who are in dispute with Southern Rail over the role of conductors.
Southern said first-class travel had been declassified across all its services, telling ticket holders they could claim back the difference.
Last week Southern Rail urged the RMT union to hold a referendum on the issue, hoping to put an end to the dispute.
Strikes also caused travel chaos last week, and more are planned throughout November and December.