The news came hours after a separate strike at ScotRail intensified with six more days of stoppages announced and a ballot to be held among other staff.
The Rail, Maritime and Transport union (RMT) said staff at Serco-run Caledonian Sleeper had voted by 85 per cent in favour of industrial action.
RMT general secretary Mick Lynch said: “Following the overwhelming vote in the recent ballot of RMT members on the Serco Caledonian Sleeper, the union will be looking at dates to put on strike action.
“Our members reasonably expected a pay rise as a reward for the hard work that they carried out in the past year during the pandemic.
"I doubt very much that the Serco shareholders have been told that there will be no dividend for them.
“This contempt towards a loyal and dedicated workforce is completely unacceptable.”
Kathryn Darbandi, Serco’s managing director for Caledonian Sleeper, said: “We are disappointed that the RMT has balloted for industrial action.
"We feel it is unjustified, given that our front line staff have received above inflation pay increases averaging 2.9 per cent each year for the past six years.
“Furthermore, even though our services have been greatly reduced, our team has remained on full pay and no one has been made redundant, despite the immense challenges posed by the pandemic.
“That makes RMT industrial action particularly frustrating at a time when we should all be united.
“Once potential strike dates are confirmed, we will update guests due to travel of any changes to their journey, and do all that we can to minimise disruption."