The move comes on top of disruption to passengers in a separate RMT conductors’ dispute over payments for working on days off, which has seen the cancellation of many ScotRail trains on Sundays since March.
RMT members – who comprise the largest part of ScotRail’s unionised workforce – are being recommended to back both a strike and taking action short of a strike “during the whole duration of the Cop26 conference”, which runs from November 1 to 12.
The union said it had been prompted by no pay offer being made by ScotRail.
However, the RMT’s decision to take soundings before deciding whether to press ahead with a strike ballot suggested its leaders may be unsure of the level of support for industrial action.
A letter to ScotRail members from general secretary Mick Lynch stated: ”Your negotiating team has met with your company council representatives to discuss this year's pay award.
"No offer has been tabled yet and the national executive committee (NEC) has instructed me to hold a referendum to ask the following two questions of you and your colleagues:
"1. Are you prepared to take strike action during the whole duration of the Cop26 conference?
“2. Are you prepared to take action short of a strike during the whole duration of the Cop26 conference?
“The NEC is recommending that you vote yes to both questions.”
ScotRail has insisted any increase must be accompanied by productivity improvements to pay for it because of the huge increase in Scottish Government support to keep services running during the pandemic.
It had planned to run extra services for people attending the conference at the Scottish Event Campus on the Clyde, just west of Glasgow city centre, which is served by the adjacent Exhibition Centre station with direct trains from across the west of Scotland.
The operator condemned RMT’s move as “reckless”.
A spokesperson said: “Industrial action is completely wrong at a time when we are seeing a gradual increase in the number of customers, but with passenger numbers at only 50 per cent of the pre-Covid level, this is not the time to put that recovery in jeopardy.
“Railway jobs are being put at risk by the reckless actions of the RMT and we are urging union bosses to think again and call off industrial action.”
In a separate dispute, train conductors are seeking extra payments for working on “rest days”, like drivers.
It followed ScotRail extending its deal with drivers’ union Aslef over the higher payments because recruitment to fill vacancies has been delayed by the Covid crisis.
The extra payments were also made to conductors until their separate agreement lapsed when vacancies were filled.
Strikes are also threatened within weeks among 200 ScotRail engineering staff, who maintain trains, after members of the Unite union voted on Wednesday for action.