The RMT union has confirmed that Calmac ferry workers will take industrial action later this month amid fears over jobs.
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 RMT union said staff feel "caught in the crossfire" of the tendering process for the Clyde and Hebrides ferry services which it says has left jobs, pensions and conditions at risk.
Members voted by more than 90% for action and have been instructed not to book on for any duty on Friday June 26 if a ship is in port. A ban on overtime is in place for June 24 and 25.
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.
"As a result of the failure to give staff the most basic assurances, we are now using the massive mandate secured in the ballot and confirming a programme of industrial action.
"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 for the first phase of industrial action."
Transport minister Derek Mackay insisted the west coast ferry service is not being privatised when he was questioned on the ongoing dispute at Holyrood yesterday.
Mr Mackay said: ''What is being tendered is a public-service contract to operate lifeline services on behalf of Scottish ministers.
''The operator would have to apply for a service specification defined by Scottish ministers and, as now, be subject to stringent contract management conditions.
''All of the vessels and ports currently in public ownership will remain in public ownership and, together with Clyde and Hebrides services, remain under public control by Scottish ministers throughout the contract."
A CalMac spokesman said: "We are aware of the latest announcement from the RMT and are very conscious of the reliance placed on our ferry services by the communities we serve and visitors to the area.
"We have scheduled discussions with our trade unions to take place on Thursday this week at which we will discuss their concerns."
Calmac and Serco have been invited to submit a final tender in December, with a view to the contract being awarded in May next year.
A Transport Scotland spokesman said: "Ministers want to protect terms, conditions and pensions for CalMac staff in the next Clyde and Hebrides Ferry Services contract, which will take effect from October 2016, and discussions between Transport Scotland and unions on how best to do that are already under way.
"Any detailed changes to the existing pension scheme are for the employers and the relevant unions to address together, and we note the process has now commenced.
"The scheme should, however, reflect the Scottish Government's principles of fairness, affordability and sustainability for public-sector pensions.
"We are disappointed at the decision to announce industrial action when discussions between the two sides are ongoing."
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