Almost 1,400 workers on Virgin East Coast, which runs services through Stevenage and Peterborough, are to be balloted for industrial action over the sacking of a colleague.
Members of the Rail, Maritime and Transport union will vote in the coming weeks on whether to take action.
The union claimed the disciplinary process over the dismissal of a worker based at Edinburgh was “flawed.”
A company spokesman said: “We’re disappointed by the announcement of a ballot by the Rail, Maritime and Transport Union as negotiations with them are still ongoing.
“We do not believe action is necessary and continue to discuss this issue with the union to avoid any disruption to customers.”