UPDATE: Caledonian Sleeper bosses respond to industrial action threat
The RMT is balloting members it says are at "breaking point"
In response Ryan Flaherty, Serco managing director of the Caledonian Sleeper, said the company is "deeply disappointed and surprised" at the move.
"We stand ready to discuss the points they have raised and their decision to proceed to a ballot is premature, not in the best interests of our employees and contrary to accepted practice," he said.
"We would urge the RMT not to disrupt the service for our guests, for some of whom this is a lifeline service."
RMT general secretary Mick Cash said earlier: "RMT has been deeply concerned over reports received from representatives in relation to the introduction of new rolling stock on the lowlander services.
"The introduction of these new trains should be a positive move for both our members and the railway industry. However, this could not be further from the truth, with the situation now reaching breaking point.
"The union hasinformed the company that it is being widely reported that stress and mental health issues are rife amongst members, as well as members having physical breakdowns and being left in tears at the end of their shift.
"The situation is clearly not helped with insufficient staffing levels, insufficient training and management being unresponsive.
"The union has informed the company that the morale of their employees has collapsed; with some unable to see how they are ever going to have anything like a good work-life balance again.
"This situation is deplorable and the response from Serco has been inadequate, dismissive and has totally broken industrial relations between our two organisations.
"RMT’s national executive committee has considered this matter and is disappointed at the company’s clear lack of care for our members.
"Accordingly, the union is now in dispute with the company and immediate preparations are underway for balloting our Serco Caledonian Sleeper members for strike action and action short of strike."
Earlier this week it was revealed that the introduction of new carriages on the route linking London, Aberdeen, Fort William and Inverness was being pushed back from June 2 to July 7 with Scottish Government transport minister Michael Matheson blaming Spanish firm CAF for being unable to supply enough carriages in time.