Passengers were warned today that ScotRail trains "would come to a grinding halt" after the Rail, Maritime and Transport (RMT) union today voted "overwhelmingly" to ban working on days off.
The action involving 3,500 staff is expected to start in about two weeks' time.
They are angry at getting paid less than drivers for working on "rest days".
Train conductors and other RMT members are paid between £112 and £140 for working their rest days compared to £300 for drivers.
RMT Scotland organiser Mick Hogg told The Scotsman: "The ballot result has voted overwhelmingly for action short of a strike.
"It will be full steam ahead as far as we are concerned.
"We will be progressing with action banning rest day working with immediate effect - subject to the 14 day formal notification."
"The likely disruption is massive.
Our railway will come to a grinding halt if overtime is banned - no question about this.
"If they don't come forward with something, it's curtains for ScotRail."
However, industry sources said such talk was "overblown" and the impact on services would be "minimal".
RMT members voted by 1,077 to 73 for action short of a strike.
They also backed strike action by 860 to 284 votes, but this is below the required threshold of 40 per cent of eligible members.
Only just over half of members voted.
Mr Hogg said the train operator was already short of a range of staff including conductors and ticket examiners, station and ticket office staff, cleaners and engineers.
He also said Sunday working was based on staff doing overtime.
ScotRail chief operating officer Angus Thom said: “We are disappointed by this result, and will continue to work with our people and their trade union representatives on this.
"ScotRail is investing more than £4 million in 140 new front-line roles to improve the work-life balance of our people and provide a better service to our customers.
"This investment means more people employed by ScotRail and will significantly reduce the need for rest day working.”
A spokeswoman for the Scottish Government's Transport Scotland agency, which controls the ScotRail franchise, said: “While industrial relations are a matter for ScotRail and the union concerned, we would encourage everyone to continue dialogue to bring this matter to a conclusion as quickly as possible for the benefit of staff and passengers alike.”