Maritime union RMT is balloting CalMac ferry staff on strike action in a dispute over jobs, pensions and working conditions.
The union said it has sent ballot papers to hundreds of members, with votes due back in a fortnight.
It has declared a formal dispute with CalMac, which it says has failed to provide RMT with satisfactory reassurances over job security, conditions of service, pensions and continuity of lifeline ferry services for remote communities and businesses.
Members are being balloted for both strike action and action short of a strike.
RMT general secretary Mick Cash said: "The clock is ticking and the union has no alternative but to begin balloting CalMac members for strike action and industrial action short of strike action, in order to secure our members' rights during this period of increasing uncertainty and insecurity.
"RMT remains available for talks."
CalMac operates ferry services between the mainland and the islands off Scotland's west coast.
A CalMac spokesman said: "We have now shared our pension reform proposals with staff and unions and look forward to meeting with them on Thursday (May 21) to discuss in detail.
"Our aim is to safeguard the pension fund by ensuring it is affordable and sustainable for years to come, while also offering value to the Scottish taxpayer.
"We appreciate any proposed amendments to pension arrangements are a matter of concern for our employees but we hope these issues can be resolved in a consensual and common-sense manner.
"We are disappointed that the RMT has decided to continue with balloting its members on potential action before we have had a chance to discuss the issues with them."
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