Transport union RMT pointed to internal Scottish Government documents which say it could happen in around 12 weeks “in an emergency”.
ScotRail is now back in public hands for the first time in 25 years after Dutch company Abellio ended its franchise three years early over criticism of its performance.
The private operator Serco has been running the Sleeper service since 2015.
RMT Secretary General Mick Lynch said: “When the Scottish Government announced last March that it would bring ScotRail into public ownership, RMT immediately urged them to do the same for the Caledonian Sleeper and reintegrate it into ScotRail, which it would never have shared may from the outset.
“A year later we find out that instead of putting the Caledonian Sleeper back on a Scottish public and integrated rail operator, the Scottish Government has inexplicably handed the private operator Serco another annual contract under which it can be paid fees that financed from public funds.
However, shockingly, internal documents confirm that not only does the Scottish Government have the necessary structures to bring the Caledonian Sleeper into public ownership, but that it would only take around 12 weeks.
“RMT is calling on the Scottish Government to sever ties with Serco and take the necessary precautions to place the Caledonian Sleeper in public ownership as a matter of urgency.
“This is the best way to offer Scottish passengers and taxpayers value for money and stability, ensuring all revenue is reinvested in improving the network rather than being paid out in fees to private operator Serco. “
The Scottish Government has been asked for comment.
It comes as unions celebrated the move to make ScotRail publicly owned and Mick Hogg, director of RMT Scotland, told Good Morning Scotland: “There will be no more private shareholders receiving dividends or profits from Scottish railways, so with that in mind we believe this is a great opportunity for Scottish Railways to receive the right investment that it deserves.”
Mr Hogg said Transport Secretary Jenny Gilruth had “checked all the right boxes on the RMT and she does not want industrial action”.
https://www.scotsman.com/news/politics/snp-ministers-urged-to-bring-caledonian-sleeper-into-public-ownership-as-scotrail-nationalised-3636917 SNP ministers urged Caledonian Sleeper to be placed in public ownership if ScotRail was nationalised