Conductors were balloted, which closed yesterday, with a majority voting for both strike action and action short of a strike, over the introduction of driver-only operation.
Union RMT said this would see 300 conductors either move to a new role, which would see them not open doors, or face voluntary redundancy.
The union warned this would threaten jobs, conditions and public safety.
RMT general secretary Mick Cash said: ‘RMT, supported by our Aslef colleagues, is wholly opposed to the attack on the safety-critical conductor grades and the threat of driver-only operation.
‘Our members have given loyal and professional service and are being rewarded by being forced into a new grade, having their role and responsibility reduced and their hard-earned terms and conditions attacked. The anger at the threat to jobs, their role and the safety is reflected in this massive vote for action.
‘This company has already axed catering services, threatened ticket offices and delivered appalling levels of customer service in their drive to milk these routes for every penny they can regardless of the impact on safety, reliability and quality.
‘The threat to the conductors is just another twist of the knife. These trains are desperately over-crowded and the conductors are the eyes and ears preventing a major tragedy on the platforms and carriages.’
A spokesman for Southern Railway, which is run by Govia Thameslink, said there would be no job cuts.
He said: ‘A strike would be unnecessary and damaging. The changes we are making to the conductor role mean there will be no job losses and no reduction in salary for any staff, whilst passengers will benefit from having more visible staff on trains.
‘We are preparing contingency plans in case a strike does go ahead but in the meantime we urge the RMT to return to the negotiating table and discuss the changes to the conductor role we are seeking to make.’
Members of the RMT union will go on strike at 11am on April 26, May 10 and May 12.