Industrial action affecting island ferry services are to be announced this week.
CalMac union RMT issued a renewed call for cast iron assurances on jobs, conditions and pensions.
The RMT fears the state-
Serco is Cal Mac's only competitor in the bidding war for a £1 billion funding package to run the ferry service.
Over 90% of the 350 union members who voted backed strikes and other industrial action.
Island ferry crews get ready for strike action
15 June 2015
RMT general secretary Mick Cash said: "RMT members on CalMac rightly feel that they are caught in the crossfire of an unnecessary and damaging tendering battle that leaves jobs, conditions and pensions hanging by a thread. That is simply intolerable.
"No lessons appear to have been learnt from the fiasco of the Northern Ferries tendering or the Scotrail franchising that is now mired in the controversy that has engulfed the winning company Abellio.
"At every turn it is the workforce and the users of these essential public services who are expected to pay the price and RMT refuses to roll over and accept that outrageous state of affairs.
"RMT wants cast iron assurances and we want them now and we are prepared to engage in meaningful talks around that agenda as we prepare to announce a first phase of industrial action."
A CalMac spokesman said: "We recognise that there are difficult issues facing our employees currently.
"We acknowledge that our trade unions wish to support the interests of their members, however, we do not believe that taking steps toward industrial action is appropriate at this stage.
"We have only just entered dialogue on the matters. That process has only just started and hasn’t had any chance to deliver progress.
"We would be concerned that a dispute now can only damage the prospects of reaching a mutually acceptable outcome. We remain committed to continuing the ongoing discussions with all trade unions."