The RMT said its members will take industrial action on Friday and Saturday because of an unresolved dispute about pay and conditions with operator Serco.
It alleges that deliberate bullying and harassment of staff has increased since talks collapsed before the start of Cop26, claiming it is a deliberate tactic to undermine workers’ morale.
RMT general secretary Mick Lynch said: “Caledonian Sleeper has had adequate time to come up with a fair pay settlement for this key group of Scotland’s rail workers as Cop26 enters its second week.
“Instead, they have kicked the can down the road and left us with no option but to go ahead with this latest phase of action from Thursday.
“We know that these strikes will close rail services in and out of Scotland but the blame for that lies with Serco and the political leadership at Holyrood.
“It’s time for all parties to take the Caledonian Sleeper workers seriously, get back around the table and give these staff at the front line of our green transport services the justice, respect and reward they deserve.”
Kathryn Darbandi, managing director for Serco Caledonian Sleeper, said: “It is very disappointing and frustrating that the RMT leadership has decided to continue with the planned industrial action this week.
“With Transport Scotland’s support, we had made a good pay offer to settle this long-running dispute, and we had been encouraged to understand that it would be positively received.
“Not unreasonably, our offer was in part dependent on the RMT calling off the strike action planned for later this week.
“Understandably, the RMT want to put the offer to their membership, which is the right thing to do and we would have been fully supportive of that process.
“However, cancelling this week’s strike action whilst we await the outcome of their ballot would have allowed us to have given our guests certainty.
“The decision to go ahead with the action means that we will only be able to run limited services on November 11 and 12, causing disruption for many of our guests, to whom we apologise.
“We know that our guests plan their journeys weeks in advance and we need to be able to give them sufficient notice to make alternative travel arrangements for their journeys between Scotland and London.
“To continue with the strike action pending the ballot is unnecessary, unreasonable and very damaging to our employees who will lose more money, our guests and the wider reputation of Scotland’s railway.”
A Transport Scotland spokeswoman said: “We welcome the discussions last week that led to a sound pay offer being tabled.
“However, it is disappointing that the RMT leadership has decided not to suspend any industrial action while they, quite rightly, conduct a vote on the deal among its members.
“This approach puts negotiations on a back footing while Serco Caledonian Sleepers Limited take time to reassess the impact of the industrial action, particularly on staff and revenue.
“This decision also has a detrimental impact for staff financially, for customer confidence while we are trying to encourage passengers back to rail, and regrettably to Scotland’s reputation during Cop26.”