Ferry workers are being balloted on whether to strike in a row over the privatisation of some services.
The Union of Rail, Maritime and Transport Workers (RMT) said that state-owned Caledonian MacBrayne (CalMac) has not given assurances that the workers' pensions and working conditions will be protected.
Scottish Government agency Transport Scotland identified five CalMac routes for privatisation: Ardrossan to Brodick, Wemyss Bay to Rothesay, Oban to Craignure, Largs to Cumbrae, and Pentland Firth which links the mainland to Orkney.
The RMT said it set out "clear and reasonable demands" to CalMac over the past two months. These include assurances from the company that terms and conditions of employment will not change and that pension provisions will be protected.
The union also demands a single joint-bargaining structure for any workers who are splintered off, to ensure pay and conditions remain standardised across the new companies in future.
CalMac is accused of "stalling" and promising meetings which "never materialised".
The union said it now has "no choice but to move to a ballot of members" for a strike.
RMT general secretary Bob Crow said: "RMT has given CalMac the best part of two months to give us the most basic assurances on pensions, working conditions and bargaining rights, and they have dragged this out to the point where we are left with no option but to ballot for strike action. And we are confident that our members will deliver a resounding mandate as they fight to protect their rights, their working conditions and their futures. The carve-up of the Scottish ferries services for private profit continues apace under the SNP administration, regardless of the impact on jobs, services and fares and, with CalMac routes next on the block, we are not going to sit back and wait for another stitch-up.
"RMT has proved, through the successful agreements with (outsourcing and services firm) Serco on the Northern Isles routes only this week, that where the workforce stands strong and united, we can protect jobs and working conditions. RMT will take whatever action is required to protect our members' working conditions and their pension rights as the vultures hover to bleed these lifeline ferry services for every penny they can extract."