The walkout will follow a two-day overtime, which may also disrupt sailings on CalMac’s 25 routes between Kintyre and Lewis.
It follows a decisive vote for action over fears for jobs, conditions and pensions as Scottish Government-owned CalMac competes with private operator Serco for the next ferry contract, of up to eight years.
Ferry workers voted by 92 per cent for a stoppage, which also equates to more than half of those eligible to vote.
Peace talks between CalMac and the RMT are scheduled for tomorrow.
The union has claimed the service is being “set up for takeover by the profiteering private company Serco”, which ministers denied yesterday.
General secretary Mick Cash said: “RMT members on CalMac rightly feel 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.”
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 at which we will discuss their concerns.”
Transport minister Derek Mackay yesterday insisted the service was being privatised.
He 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.”