Members of the Rail, Maritime and Transport union (RMT) voted by 80-13 for a ballot on taking industrial action over pay.
The union said operator Serco had insisted that any wage increase must be funded by productivity improvements, under emergency Scottish Government funding to keep the service going during the Covid crisis when passenger numbers have plummeted.
Staff were asked: “Do you want the RMT to ballot members for industrial action over this years pay discussions?"
Mick Hogg, the unions’ Scottish organiser, said: “It is an overwhelming result for action.
"The next step will be a formal ballot for action on pay for 2021.
“Serco has taken the same line as ScotRail, via Transport Scotland – agree nothing that increases your cost.
"Therefore on pay, they have offered nothing.”
The RMT and ScotRail’s three other unions are also staging consultative ballots over a similar pay situation there.
Kathryn Darbandi, Serco’s managing director of Caledonian Sleeper, said: “It is deeply disappointing that the RMT is voting for industrial action when our focus should be on supporting the recovery from the pandemic.
"It is particularly frustrating given that all our employees have continued to be employed and on full pay throughout the pandemic, despite a reduced service being in place for most of the last year.”
The threat of disruption comes two and half weeks before travel restrictions across mainland Scotland are expected to be scrapped.
However, the Scottish Government has yet to announce whether non-essential journeys will also be permitted to England, such as on the Sleeper.