Rail workers on the Caledonian Sleeper service between Scotland and London will start voting next week on whether to go on strike in a dispute over safety.
Members of the Rail, Maritime and Transport union employed by Serco will decide by December 15 whether to launch a campaign of industrial action.
The union would have to give seven days notice of any action, raising the threat of a strike over Christmas.
The union said despite a number of meetings between RMT negotiators and senior company management over the last few months, progress has not been made on a raft of issues including claims of defects in rolling stock.
RMT general secretary Mick Cash said: "The poor service to passengers, in conjunction with the company's failure to address the issues raised by RMT representatives and officials in recent talks, has now reached the point where the union has decided enough is enough and we will now proceed to ballot for industrial action.
"Our members have been unhappy with Serco's management of the iconic service since the very early days of them taking on this 15-year franchise."
Serco, which took over the franchise earlier this year, has said it is working hard to improve the service.
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