A transport union is preparing a strike ballot in a row over train driver cover on ScotRail services.
The TSSA union has written to the railway operator warning of a "near complete breakdown of industrial relations" regarding the dispute.
The union claims ScotRail changed contracts for driver team managers, responsible for training drivers and monitoring the newly qualified, so they have to cover train drivers' shifts.
The change in terms to cover driver absences included a salary increase and a rest day working payment of £375.
The TSSA said this has implications for other workers but claims it was excluded from negotiations on this and other issues.
TSSA general secretary Manuel Cortes said: "ScotRail wouldn't need to force driver train managers to cover shifts if they had employed enough drivers in the first place.
"As always Abellio are trying to do things on the cheap and it's blowing up in their face.
"ScotRail has systematically and deliberately excluded our union from talks.
"We believe this is a cynical and deliberate attempt to undermine our union and deny our members their right to representation and a voice in their workplace.
"Our members are rightly furious. They are ready and determined to vote for strike action and I will have no hesitation in sending out the ballot papers if ScotRail don't come back to the negotiating table with us."
He called for Transport Secretary Michael Matheson to intervene.
In a letter outlining members' concerns and plans to ballot for industrial action, TSSA Scotland organiser Gary Kelly said there had been "a near complete breakdown of industrial relations" between the union and ScotRail.
A ScotRail spokesman said: "We've been hiring qualified and trainee train drivers to help us make Scotland's railway better.
"While this recruitment and training takes place, we have agreed that our driver team managers will drive trains to avoid cancellations.
"The investment we are making to recruit more drivers is helping us significantly to enhance the day-to-day running of services for our customers."
A Transport Scotland spokeswoman said: "While industrial relations are a matter for ScotRail and the union concerned, we would urge all parties to continue discussion to resolve this dispute at the earliest opportunity."