The RMT union has confirmed its members working for ferry operator Caledonian MacBrayne will go ahead with industrial action this week after talks failed today.
Leaders said the move is part of a fight to defend jobs, conditions and pensions.
The action will go ahead from Wednesday when a ban on working overtime begins, with 24 hours of strike action on Friday.
Publicly-owned CalMac and private firm Serco are bidding for a new contract for services to the west coast islands which are currently operated by CalMac.
The union said staff feel "caught in the crossfire" of the tendering process for the Clyde and Hebrides ferry services.
Members have voted by more than 90% for action in a ballot.
General secretary Mick Cash said: "The talks with CalMac have not made significant progress despite the best efforts of our negotiators and as a result the action goes ahead from Wednesday.
"It is extraordinary that rather than standing up for Scotland's lifeline ferry services, those who hold political power have resorted to ludicrous arguments about what does and doesn't represent privatisation. That is a kick in the teeth for loyal and hard-working staff who have given long hard years of service at CalMac.
"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.
"RMT wants cast iron assurances and we want them now and we are committed to engaging in meaningful talks around that agenda as we kick off the first phase of industrial action."
CalMac's managing director, Martin Dorchester, said: "We are disappointed that our dialogue with the RMT to avert industrial action this week has broken down.
"Our offer to provide assurance that we would include a commitment on no compulsory redundancies in our bid submission in return for the RMT calling off the industrial action planned for later this week was rejected. We were also willing to work together with both the RMT and TSSA over the coming days to agree a form of words in our collective agreements which would provide greater protection to employees around terms and conditions of employment.
"Unfortunately this was also rejected. Failure to keep these discussions alive will be a disappointment, not just to those who will be losing pay this week, but to potentially thousands of travellers of who will be disrupted. The threat of strike action also has major potential to damage the economies of the communities we are part of.
"We are currently working on alternative timetables and contingency plans and will be doing all we can to ensure that disruption to these lifeline service are kept to a minimum.
"However, the travelling public needs to be aware that disruption is inevitable and that they should consider careful whether their trip is essential before travelling later this week.
"Our door remains open. We urge union representatives to re-enter dialogue with us as soon as possible and to give urgent consideration to our offer of no compulsory redundancies and protection of existing generous terms and conditions. Our aim is to do everything possible to avert any industrial action which will impact our ability to maintain the lifeline services we provide."
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