The COP26 summit in Glasgow is facing gridlock after rail workers voted for strike action during the international event next month.
The RMT, which represents staff and is in dispute with ScotRail, announced a yes vote of 84%.
It comes as refuse workers also voted for strike action during the international conference.
After the ballot result was confirmed, the RMT said the action could take place during COP26 – a move that would give the union huge leverage.
RMT General Secretary Mick Lynch said: “Today’s ballot has been forced on us by the deliberate time wasting by Scotrail and Transport Scotland.
“This issue should have been sorted by now and we should have had a fair and just offer that recognises the hard and important work of our members.
“The result will be considered by our Executive who will decide the next steps but the company should be taking full note of this massive majority for action and we remain available for talks.”
In a tweet, the GMB trade union in Glasgow said of the vote by refuse workers: “In Glasgow we have reached the legal threshold in our Industrial Action Ballot.
“96.9% voted in favour of Industrial Action in response to the latest pay offer from @COSLA.
“Unless there is an improved offer on Monday we will be taking action in Glasgow during COP26.”
A ScotRail spokesperson said: “It’s extremely disappointing that the RMT have opted to continue with this highly damaging strike action, particularly when a pay offer, negotiated over several weeks, has been made to the trade unions.
“We’re seeing customers gradually return to Scotland’s Railway, but the scale of the financial situation ScotRail is facing is stark.
“To build a more sustainable and greener railway for the future and reduce the burden on the taxpayer, we need to change. All of us in the railway – management, staff, trade unions, suppliers, and government – need to work together to modernise the railway so that it is fit for the future.”
Labour MSP Neil Bibby said: “COP26 is falling into sheer chaos as the SNP’s industrial relations collapse.
“It is an embarrassment to Scotland that the SNP can’t even keep the trains running when the world’s eyes are upon us.
“This conference is supposed to be about preventing environmental catastrophe – and the SNP are setting the stage by letting trains grind to a halt and litter fill the streets.
“Rail workers were on the front line right through the pandemic and the way they have been treated in return is disgraceful.
“The SNP must intervene to deliver a fair pay deal before their contempt for workers brings COP26 to its knees.”
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