The rail firm is under unprecedented pressure from two trade unions: joining forces with the Rail, Maritime and Transport union (RMT), members of drivers union Aslef will walk out for nine days starting next Tuesday.
Whereas previously Southern has managed to limp through RMT industrial action, the two-pronged attack will bring services to a standstill.
But after an injunction to halt the Aslef action was yesterday thrown out by the High Court, Southern has lodged an urgent application at the Court of Appeal against the ruling.
However, given Southern has not been allocated a date for the appeal hearing to be heard, time is running out before the Aslef action commences on Tuesday.
Charles Horton, chief executive of GTR, said:
This industrial action is having a severe and significant impact on our ability to run our train services and causing massive disruption to the 500,000 passengers who travel with us every day.
We were granted permission yesterday by the judge to make an urgent appeal and we have a duty to our passengers to do all we can to prevent the wholly unjustified industrial action continuing.
Our passengers have suffered months of travel misery and we call again on the unions to call off their action and work with us to find a resolution to their dispute.
The dispute with the RMT has dominated 2016, putting hours onto commuters' daily journeys and costing the capital's businesses mid-boggling amounts in lost working hours.
After cutting services earlier in the year, Southern still managed to run a skeleton service during multiple strikes by the RMT.
However, one key element of flexibility the rail network depends on – the ability of staff to work overtime – was withdrawn this week as Aslef banned drivers from working extra hours.