The RMT said its members working on the sleeper service are taking industrial action on Friday and Saturday because of an unresolved dispute about pay and conditions with operator Serco.
Mike Lynch, RMT General Secretary, said: "It's a scandal that Serco and their political paymasters in the Scottish Government are leaving the sleeper staff out in the cold.
"There will be no sleeper services back to London from Cop26 and the responsibility for that lies fairly and squarely with the company and those holding the purse strings.
"There can be no climate justice without workplace justice and it's time for a fair deal for the Caledonian Sleeper staff."
Mr Lynch said the union remained willing to take part in further talks.
Kathryn Darbandi, Managing Director for Serco Caledonian Sleeper, said: “This is incredibly frustrating.
“We have worked closely with Transport Scotland to find a way of funding the proposed inflation-based pay offer and we were led to believe by the RMT that they would recommend this very good offer to their membership. Combined with the 2.75% awarded last year, this offer would have given our employees a total pay rise of 4.15% since April 2020, which is as good as any pay increase in the sector.
“Our offer was exactly what the RMT had requested and supported locally last week. However, it is now very clear that the RMT has not supported or recommended the offer to their membership, and they have now voted not to accept it and bring this damaging dispute to an end.”
On Wednesday, the union said the most recent offer of a 1.4 per cent rise from the company, which is below both inflation and a deal agreed for Scotrail workers, had been overwhelmingly rejected by members.
Pickets were out at key locations of the route overnight, with lines set up in London, Glasgow, Edinburgh and Inverness.
Jeremy Corbyn, the former Labour leader, also joined the picket line in Glasgow and spoke in support of the strike action.
In response to the strikes, Serco said it will be cancelling all Highland services on November 11 and 12, while the Caledonian Sleeper service will run southbound from Glasgow and Edinburgh to London Euston on Friday and return northbound the following day.
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