Caledonian MacBrayne workers have voted by two to one in favour of strike action after a ballot of Rail, Maritime and Transport (RMT) union members.
Industrial action will take place on 23 March
The union - which represents 400 CalMac staff - will now hold a 12-hour strike on 23 March on services including the Clyde and Western Isles routes.
The RMT is campaigning against the opening up of west coast ferry services to competition.
CalMac said it was "extremely disappointed" with the outcome.
A spokesman voiced hopes that union leaders would take into account that strike action would not be in the best interests of the communities served.
The company hopes talks can be held and the strike averted but accepts that if the action does go ahead it will cause widespread disruption.
However, the smaller ferry services are likely to be unaffected by the move as they are generally not operated by RMT members.
There was a 59% turnout in the ballot, with union members voting by 155 to 81 in favour of industrial action.
General secretary Bob Crow said: "After three weeks' silence from the company, I hope they will now see fit to get around the table with us to negotiate guarantees for the terms and conditions of the people who work hard to provide these lifeline ferry services."
The Scottish Executive said that European rules require it to allow private firms to bid against state-owned CalMac for the chance to operate the routes.
Consultation on the commercial tender process ends on Wednesday.
Transport Minister Nicol Stephen will then decide whether to offer the routes as a deal to the lowest bidder.
CalMac would bid for what would be a six-year contract.
'Dereliction of duty'
Whichever operator wins, the fleet will still sail in the widely-known black and white colours, even if it has different crews and management.
The Scottish National Party's transport spokesman Fergus Ewing said the strike was entirely preventable and placed the blame at the feet of the executive.
He said: "Instead of responding to the democratic will of the Scottish Parliament, the transport minister has failed to pursue alternatives to putting vital ferry services to tender.
"This inaction has led directly to this strike, and this dereliction of duty is unforgivable.
"I call on the minister to act now in taking his case to the commission and fight for the future of Scotland's ferry services."
The union represents 400 CalMac employees
The Scottish Socialist Party also joined in the condemnation of the government.
Its national convenor Colin Fox pledged his party's support for the CalMac workers and said: "They have the complete backing of the SSP in their fight to defend jobs, pay, conditions and pensions.
"The SSP has done everything we can in parliament to back the CalMac workers and we will continue to do so alongside practical solidarity by our members on the ground and in the RMT.
"By putting west coast ferry services to competition the executive is going against the democratic will of the Scottish people expressed in parliament by a vote not to put the routes out to tender."
At the same time as the strike decision was taken, the Nationalists held talks with European Commission officials on the company's future on Monday.
MEP Alyn Smith was joined at the meeting by Strathclyde University Professor Neil Kay, an expert on the issue of CalMac tendering.
Prof Kay has proposed an alternative to the privatisation plans.
Mr Smith said: "This saga has been dragging on for years. It is time the situation was remedied.
"Those who rely on vital ferry services for transport, supplies and for employment deserve an end to the uncertainty created by the executive's plans."
He added: "Communities do not want to see these lifeline services put out to tender, but want to know that they will have a ferry service they can rely on in years to come.
"There is no need for Caledonian MacBrayne services to be put out to tender, and much of the way the Scottish Executive has handled the CalMac issue is wrong-headed and backwards."