Workers on the new South Western Railway franchise are to be balloted for strikes in a dispute over the role of guards.
Members of the Rail, Maritime and Transport union (RMT) at the operator, which only recently took over the running of services across the south west, including from London Waterloo, will vote on whether to launch a campaign of industrial action.
The union said the company, a consortium of the First Group and Hong Kong-based MTR, had failed to give assurances over the future of guards.
The RMT said it had spent months since First/MTR won the franchise from Stagecoach trying to get a "water-tight pledge" on the future of guards, claiming it had been met with a "barrage of spin and prevarication".
Mick Cash, RMT general secretary, said: "RMT has given repeated opportunities for First/MTR to give us the assurances we have sought over the future role of the guards on their trains throughout the length of the new South Western franchise.
"They have refused to give us those guarantees and it is that failure which leaves RMT with no option but to declare a formal dispute with the company and to move towards a ballot for action.
"This dispute can be easily resolved if the company are prepared to stick to existing agreements, give staff and passengers alike a guarantee of a second, safety-critical member of staff on all current services, and an assurance that safety and access are the absolute priority.
"Guards on SWR have been hailed as 'legends' in company publicity material. It is time for First/MTR to stand by those legends and to withdraw the threat to throw them off the trains.
"That is the message that RMT will be taking out to the fare-paying public as we step up our campaign."