The Transport Salaried Staffs’ Association (TSSA), which represents 100 ticket office staff and managers at CalMac Ferries, will ballot workers next week amid fears that the Scottish Government is preparing to hand the Clyde and Hebridean ferries service to Serco.
Ministers say they are forbidden from favouring one bidder over another under European competition regulations.
RMT members at CalMac began three days of industrial action yesterday, with two days of work-to-rule followed by an all-out strike planned for tomorrow.
TSSA will begin a 10-day ballot of staff for industrial action on July 3.
Manuel Cortes, general secretary of the TSSA, said: “The Scottish Government is simply not listening to the unions or the general public which wants this service kept in public hands.
“We want long-term guarantees on job security for our members and a recognition that the interests of the local communities will come first, not last.
“This plan is wrong-headed and unnecessary. The SNP must not betray the people of west Scotland by selling off this vital transport link.”