Thousands of ScotRail train conductors and other staff are to be balloted for industrial action after being told their pay would not increase this year.
The RMT move followed emergency Scottish Government funding for ScotRail being extended until January.
The union did this did not include a pay rise, which should have been agreed in April.
A letter issued by the union stated: “Negotiations have taken place with the company to discuss the matter of your pay rise.
"Unfortunately the latest talks have proved to be fruitless and your union has been advised that the extension of emergency measures agreement until January 2021 implies that you will not receive a pay rise in 2020.
"Your national executive committee has considered the matter further and it has declared that a dispute situation now exists between our organisations.
"Necessary preparations for a ballot for industrial action are being made.”
The RMT urged members to vote in favour of both strike action and action short of a strike.
‘Treated with contempt’
Scottish organiser Mick Hogg said: “We are key workers and have been more than patient.
"1 April was our pay anniversary date, but we agreed reluctantly to pause pay talks.
"We are now being treated with contempt and our members have had enough.”
Under a separate two-year deal agreed with ScotRail last year, train drivers’ union Aslef will see its members’ annual pay increase by 3 per cent from £50,000 to around £51,500 next month.
Gerry Skelton, ScotRail human resources director, said: “ScotRail delivers well-paid, good quality jobs.
"While many other parts of the economy have closed down and laid off thousands of staff, the jobs, salaries, and conditions of ScotRail staff have been unaffected.
"We are proud we’ve been able to protect our people during these very challenging times and believe the delay in any pay negotiations is a small price to pay for the peace of mind and financial stability we can give our colleagues and their families.
"It’s very disappointing the RMT doesn’t appear to recognise or welcome the job security that has been provided.
“With the emergency measures agreement in place, we are in regular dialogue with the Scottish Government and trade union colleagues about when it would be right to begin pay discussions in a responsible manner over the coming months.”
News of the RMT ballot comes hard of the heels of the union announcing a series of strikes among Caledonian Sleeper staff over rest areas on trains during extended shifts.
Two 48-hour walkouts are due to take place from Sunday 4 and Sunday 11 October, along with an overtime ban on other days.
Caledonian Sleeper is looking at whether it will be forced to make changes to services if the action goes ahead.
The dispute was called after the RMT said staff were denied access to spare cabins during shifts that could last more than 16 hours.
Operator Serco has drafted in a fatigue expert.
A further meeting with the union is scheduled for next week.