Southern Railway conductors balloted over strike action
Nearly 400 Southern Railway conductors are to be balloted on industrial action over rotas and "staffing shortages".
The RMT union said the operator had failed to deal with rostering issues and there were insufficient staff.
The ballot, for strike action and action short of a strike, will open next week and close on 15 December.
South Railway, which operates in London, Sussex, Surrey, Kent and Hampshire, said it was making "every effort to resolve this issue".
RMT general secretary Mick Cash said staffing shortages on Southern were "a disgrace".
"This is a vital and lucrative franchise where commuters pay thousands of pounds a year to travel on services that are chronically under-staffed and RMT members have had enough of it.
"We will be explaining to the travelling public, who are also the victims of this corner-cutting policy, why we are being forced down this road and we will be asking them to assist us in putting pressure on the company."
A spokesman for Southern Railway said: "We have been talking to the union about its aspirations regarding terms and conditions, and have made some suggestions about how these might be achieved.
"Unfortunately, the RMT believes we should meet their demands in full.
"We are making every effort to resolve this issue with the RMT to avoid the unnecessary disruption it would cause our passengers should the ballot lead to industrial action."
He added: "We employ sufficient conductors to run the service and have a continual programme in place to recruit and train conductors to counter the effects of staff turnover."