The result of a strike ballot among Southern Railway drivers will be announced today, raising the threat of travel chaos in the run up to Christmas.
Members of the drivers' union Aslef have been voting on whether to launch a campaign of industrial action in a dispute over driver-only trains.
The union's executive will meet to discuss the outcome of the ballot before deciding the next move.
The row is separate from the long-running dispute between Southern and the Rail, Maritime and Transport union over changes to the role of conductors.
Aslef accused the company of wanting to impose changes rather than reach an agreement.
General secretary Mick Whelan said: "Our trade dispute with the company is that there should be no introduction and/or extension of new driver-only operated routes on Southern without the agreement of Aslef.
"We have genuinely sought to reach a compromise with Southern. We have always been prepared to talk to the company and we have always been of the view that it should be possible to do a deal, but it takes two to tango and the company has not been prepared to negotiate.
"They have dug in their heels and forced us to ballot our members."
Aslef will have to give seven days notice of any industrial action.
Members of the RMT have staged a series of strikes on Southern in recent months, with three-day walkouts due to start on December 6, 22 and 31.
Southern services are also being disrupted on other days because of issues including a shortage of staff.