Almost 1,400 workers for Virgin Trains East Coast are to be balloted on whether to take industrial action over the sacking of a colleague.
Members of the Rail, Maritime and Transport union claim the company's disciplinary process was "flawed".
The case involves a "missing Kindle" from a train the man in question was working on, and "inconclusive" CCTV evidence, the union said.
A spokesman for Virgin Trains said the company was "disappointed" by the news.
"We do not believe action is necessary and continue to discuss this issue with the union to avoid any disruption to customers," he added.
The union said it believed there were several reasons why the man, who was based in Edinburgh, should not have been sacked and called on Virgin Trains to "reinstate" him.
The union claims not every member of staff on duty was interviewed about the case and CCTV evidence was inconclusive and has been destroyed.
The union will start distributing voting papers to all those in the company working in onboard catering, guards, and at station and ticket offices on Monday 12 October. The ballot will close on Tuesday 27 October.
Services on the Virgin Trains East Coast route currently run from London King's Cross to Inverness and Aberdeen, serving locations such as Edinburgh, Glasgow, Peterborough, Doncaster, Leeds, York and Newcastle.